Ophiolitic Rocks Research Articles

The provenance of Devonian sandstones from Shetland: a Sm-Nd and trace element study

Synopsis A Sm-Nd isotope and trace element study has been performed on a selection of four whole rock samples of Devonian sediments, two impure/banded quartzite boulders and two unfoliated granite/granophyre boulders from a Devonian conglomerate. Included in the study are also three samples of Caledonian metasediments (from the Whiteness, Clift Hills and Yell Sound Divisions) and one Archaean tonalite gneiss (the Wilgi Geos Group) from Shetland. The Devonian sediments and the granite boulders have relatively low Nd model ages of 1.0 to 1.1 and 1.3 to 1.5 Ga, respectively, which suggests the presence of Caledonian juvenile material in both of these rock types. The Proterozoic metasediments have Nd model ages in the range 1.8 to 2.5 Ga, while the tonalitic gneiss has a high Nd model age of 2.9 Ga. The Caledonian metasediments form two compositionally contrasting groups where the Yell Sound and Clift Hills Divisions and the two impure quartzite boulders all have relatively low time-corrected 143 Nd/ 144 Nd ratios and low Nb-and Sr-contents relative to the Whiteness Division. These two groups of Caledonian metasediments together with representative Caledonian ophiolite rocks define a compositional field of time-corrected 143 Nd/ 144 Nd ratios and Nb and Sr contents, within which plot the compositions of the Devonian sediments and the granite/granophyre boulders. Cr is generally immobile at surface and low-grade metamorphic conditions, but is less suitable as a provenance indicator due to the wide variations in the Cr-content in Caledonian juvenile rocks.

Provenance of sandstones from the Wakino Subgroup of the Lower Cretaceous Kanmon Group, northern Kyushu, Japan

Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high‐grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote‐amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle‐derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted.

Geochemical and mineralogical characteristics of Fe–Ni- and bauxitic-laterite deposits of Greece

Contiguous vertical sample profiles from Ni-laterite deposits with in situ features (Kastoria, Profitis Ilias, and Tsouka) and allochthonous Ni- and bauxitic-laterite deposits in contact with basement limestone (Nissi) and bauxitic-laterites lying on peridotite (Parhari) were analyzed for major, trace (including Th and U), rare earth and platinum-group elements (REE and PGE, respectively). In addition, minerals with emphasis to chromite grains found as residual components in these laterites, inherited from the ophiolitic parent rocks, were analyzed by microprobe. Low Al 2O 3, TiO 2, REE, Th and U contents are common features of the Kastoria, Bitincka and Tsouka deposits. In contrast, elevated REE contents are present in the karst-type bauxitic- and Ni-laterite ores of Nissi and bauxitic-laterites of Parhari. The bauxitic-laterite deposit of Nissi attains REE contents of thousands parts per million in samples from the contact between the lowest part of the bauxitic-laterite and the footwall limestone. Highest contents of Th and U are found in the bauxitic-laterites, with Th ranging from 4 to 28 ppm and U from 4 to 66 ppm. In general, increasing Al contents are accompanied by elevated Ti, REE, Th and U contents at the Parhari and Nissi laterite deposits. Goethite, is the dominant mineral in all Ni-laterite profiles studied, while boehmite co-existing with goethite is common in the bauxitic-laterites at Nissi and Parhari. Goethite exhibits variable Al contents, while the Al/Fe ratio increases towards the top of the profiles. The PGE concentrations are generally low, ranging from less than 100 ppb to a few hundred parts per billion. The lowest values — lower than in the bedrock — were recorded in the saprolite zone. A certain enrichment in Pt (up to 48 ppb), Pd (7 ppb), and Au (16 ppb) is recorded in the reddish altered peridotite overlying the saprolite zone at Profitis Ilias. The Fe–Ni ore overlying the reddish altered peridotite has the highest Os values (14 ppb), Ir (32 ppb), Ru (66 ppb), Rh (20 ppb), Pt (86 ppb), and Pd (186 ppb). Gold contents are below 36 ppb. An increasing Pt/Pd ratio from 3.0 in Fe–Ni-laterites to 6.0 in bauxitic-laterites is apparent. Both whole rock compositions and mineral chemistry of laterites indicate that major controlling factors of the composition of the bauxitic-laterites are the conditions during transportation/deposition of the weathered material and during diagenesis/metadiagenesis stage rather than parent mafic ophiolitic rocks. The comparison between the primary composition of chromite in the saprolite zone and the overlying Fe–Ni ore may provide evidence for the discrimination between Fe–Ni ore linked to in situ weathering and ore derived by transportation to some extent of clastic and chemical material.

Provenance of sandstones from the Lower Cretaceous Sasayama Group, Inner Zone of Southwest Japan

Abstract Modal analysis and phase chemistry of sandstones of the Lower Cretaceous Sasayama Group, Southwest Japan, indicate that the detritus were derived from heterogeneous sources that include medium- to high-grade metamorphic, mafic–ultramafic, and acidic magmatic rocks. Garnet geochemistry suggests their derivation from epidote-amphibolite to granulite facies metamorphic rocks. Sphene and chlorite geochemistry suggests their derivation from mafic rocks. Chemical compositions of chromian spinel indicate input from Alpine-type peridotite. The chemistry of the chromian spinel further suggests that the Alpine-type peridotite source was most probably formed under an oceanic back-arc basin setting rather than mid-oceanic ridge setting. Chemical compositions of chromian magnetite indicate that the Alpine-type peridotite provenance was subjected to metamorphism of up to the lower amphibolite facies prior to erosion and transport. The metamorphic and granitic rocks of the Hida–Oki Terrane, ophiolitic rocks of the Sangun–Renge Terrane, and rhyolitic rocks from uncertain terrane (Akiyoshi?) are most probably the major source of detritus to the Sasayama basin. The basin is most likely a strike–slip basin that formed along the eastern margin of the Asian continent.

FORMATION AND EMPLACEMENT AGES OF THE SSZ-TYPE NEOTETHYAN OPHIOLITES IN CENTRAL ANATOLIA, TURKEY: PALAEOTECTONIC IMPLICATIONS

Isolated outcrops of dismembered ophiolitic rocks occur as allochthonous units (Central Anatolian Ophiolites, CAO) in the Central Anatolian Crystalline Complex, which represents the metamorphosed passive northern edge of the Tauride- Anatolide Platform. Both stratigraphic observations and chemical data indicate that the CAO formed in a supra-subduction zone (fore arc) setting within the Vardar-Izmir- Ankara-Erzincan (VIAE) segment of the Neotethys. The epi-ophiolitic, sedimentary cover of the Sarikaraman Ophiolite, which is a representative unit of the CAO, is characterized by epiclastic, volcanogenic, deep-sea sediments, with the latter being intercalated with debris-flows. The most significant planktonic foraminiferal association found in the lowest pelagic member of this unit indicates an early-middle Turonian to early Santonian age. K/Ar data (81-65 Ma) collected from post-collisional granitoids intruding both the basement rocks and the Sarikaraman Ophiolite imply a post-early Santonian to pre-middle Campanian emplacement age for the CAO. The high volume of epiclastic volcanogenic sediments that occur alternated with both the pelagic limestones and the radiolarites of the CAO suggests that the depositional setting was a marginal sea adjacent to a volcanic arc. The occurrence of debris-flows indicates also that the depositional basin was affected by tectonism, that resulted in major slides and mass-gravity reworking of preexisting units and of arc-derived volcanics and sediments. The stratigraphy and the age of formation and obduction of the CAO were compared with other supra-subduction zone type ophiolites of both the eastern (Turkey) and the western (Greece) parts of the VIAE segment of the Neotethys. This comparison indicates that the western intraoceanic subduction is definitely older. Accordingly, we surmise that important transform faults were separating the continental micro-plates within the VIAE-ocean (e.g. the Tauride-Anatolide microplate) during the closure of this oceanic branch.

Suprasubduction Zone Origin of the Pozanti-Karsanti Ophiolite (Southern Turkey) Deduced from Whole-rock and Mineral Chemistry of the Gabbroic Cumulates

Abstract The Pozanti-Karsanti Ophiolite Complex is situated in the eastern Tauride Belt and represents a remnant of the Mesozoic Neotethyan Ocean. It consists of three distinct nappes: (1) an ophiolitic mélange; (2) a metamorphic sole; and (3) ophiolitic rocks. The oceanic lithosphere section of the Pozanti-Karsanti Ophiolite comprises mantle tectonites, ultramafic-mafic cumulates, isotropic gabbros, sheeted dykes and basaltic volcanic rocks. These units are cut by isolated microgabbro-diabase dykes at all structural levels. New results are presented on the whole-rock and mineral chemistry of the gabbroic cumulates. Well-layered, low-Ti gabbroic cumulates, showing adcumulate to mesocumulate textures, are represented exclusively by gabbronorites. The mineral chemistry of gabbronorites from the Pozanti-Karsanti Ophiolite indicates that these cumulate rocks have been produced by the low-pressure crystal fractionation of basaltic liquid. Magnesium numbers (Mg-numbers) of clinopyroxene, orthopyroxene and amphibole range from 89 to 73, 80–66 and 80–72, respectively. Plagioclase compositions range from An 94 to An 84 . The coexistence of calcic plagioclase, magnesian clinopyroxene and orthopyroxene indicates that the cumulate gabbronorites from the Pozanti-Karsanti Ophiolite were formed in an arc environment. The covariation of Al 2 O 3 and Mg-numbers of both clinopyroxene and orthopyroxene show features typical of low-pressure igneous intrusions such as the Skaergaard and Tonsina Complexes, but differ from the high-pressure ultramafic cumulates found in the same arc. The cumulate gabbronorites probably represent shallower levels in the arc which were subsequently juxtaposed against deeper level ultramafic cumulates either during accretion or later faulting.

Formation and emplacement ages of the SSZ-type Neotethyan ophiolites in Central Anatolia, Turkey: palaeotectonic implications

Isolated outcrops of ophiolitic rocks, termed the Central Anatolian Ophiolites, are found as allochthonous bodies in the Central Anatolian Crystalline Complex, that represent the metamorphosed passive northern edge of the Tauride–Anatolide Platform, central Turkey. In terms of pseudostratigraphic relationships of the magmatic units and their chemical designation, the Central Anatolian Ophiolites exhibit a supra-subduction zone (fore-arc) setting within the Vardar–İzmir–Ankara–Erzincan segment of the Neotethys. The epi-ophiolitic sedimentary cover of the Central Anatolian Ophiolites is generally characterized by epiclastic volcanogenic deep-sea sediments and debris flows intercalated with pelagic units. The richest and most significant planktonic foraminiferal association recorded from the lowest pelagic members infer a formation age of early–middle Turonian to early Santonian. K/Ar ages of post-collisional granitoids (81–65 Ma) intruding the basement rocks as well as the Central Anatolian Ophiolites suggest a post-early Santonian to pre-middle Campanian emplacement age. The marked high volume of epiclastic volcanogenic sediments intercalated with the pelagics of the Central Anatolian Ophiolite is suggestive of rifting in a marginal sea adjacent to a volcanic arc. Penecontemporaneous tectonism is reflected in repetitions in the stratigraphy and in debris flows, which result from major slides and mass-gravity reworking of pre-existing units and of arc-derived volcanics and sediments. Correlating the rock units and formation/obduction ages of the Central Anatolian Ophiolites with further supra-subduction zone type ophiolites in the eastern (Turkey) and western (Greece) parts of the Vardar–İzmir–Ankara–Erzincan segment of Neotethys we conclude that the intraoceanic subduction in the east is definitely younger and the closure history of this segment is more complex than previously suggested. Copyright © 2000 John Wiley & Sons, Ltd.

Formation of mélanges in the Indus Suture Zone, Ladakh Himalaya by successive subduction-related, collisional and post-collisional processes during Late Mesozoic-Late Tertiary time

Abstract The classic Indus Suture Zone in western Ladakh includes two zones (southern and northern) of highly dismembered rocks, that in the past were widely interpreted as ophiolitic mélanges, created mainly by subduction/accretion processes. The ‘ophiolitic mélange’ was reported to include chaotically distributed blocks of ophiolitic rocks (e.g. serpentinite, gabbro, basalt) and sedimentary rocks (chert, limestone) set in a matrix of deep-sea clastic sediments. This accretionary hypothesis is tested in this paper and found to be inadequate. Units formed simply by oceanic subduction-accretion (i.e. local mud-matrix mélange) are minimal (<1% by volume). In reality, the southern and northern mélange zones are the end products of complex multi-stage tectonic processes, involving subduction (mid-Late Cretaceous to Early Tertiary), initial emplacement (latest Cretaceous), collision (Palaeocene-Eocene) and post-collisional (Late Tertiary) stages. Important components of the mélange as a whole include thrust sheets and broken formation of relatively coherent volcanic-sedimentary successions related to the North Indian passive margin (Karamba and Lamayuru complexes), also mid-Late Cretaceous oceanic arc-type volcanics and volcaniclastic sediments (Dras arc complex). Dismembered serpentinite thrust sheets, cut by swarms of (subduction influenced) diabase dykes, most likely record detached oceanic basement related to the oceanic Dras arc complex. Associated serpentinite was injected along tectonic contacts and into adjacent units during collisional and post-collisional deformation, locally forming serpentinite mélange. Post-collisional, Early-mid-Tertiary non-marine coarse clastic sediments (Indus Group) unconformably overlie various units (serpentinite, volcaniclastics, blueschists) in different locations, and were later thrusted and folded within the southern and northern mélange zones in response to regional Late Tertiary tightening of the suture. As a result, backthrusting reversed the original thrust stacking order, placing the originally most southerly units at the highest structural level. The Indus Suture Zone ‘ophiolitic mélanges’ in western Ladakh are, thus, mainly not ophiolitic (i.e. dismembered oceanic crust), or mélange in entirety (i.e. chaotic block), but are instead dominated by disrupted thrust sheets and broken formation of the Mesozoic Lamayuru-Karamba continental margin and the Upper Cretaceous oceanic Dras arc complex, with only minor preserved remnants of units formed by subduction/accretion (e.g. mud-matrix mélange; oceanic exotics). The southern and northern mélange zones reflect the existence of several N-dipping subduction zones active in latest Cretaceous-Palaeogene times. Although only minor volumes of accreted oceanic material (oceanic lithosphere and trench-type sediments) are preserved within the Indus Suture Zone in western Ladakh, additional accreted material was bulldozed further south onto the Zanskar continental shelf and is now preserved as mélanges beneath the Spontang ophiolite. Similar mélange is also preserved in eastern Ladakh. Subduction zones evolved into a major interconnected shear zone (suture) during Early Tertiary (54–50 Ma) collisional deformation, dismembering upper (oceanic arc) and lower plate (continental margin) units to produce most of the southern and northern mélange zones. After suturing and initial deposition of non-marine coarse clastic cover sediments, inherited weakness zones within the Indus Suture Zone were exploited, increasing disruption during Late Tertiary regional backthrusting. The methods used here in the analysis of the Indus Suture Zone mélange could well prove to be useful elsewhere in the Himalaya and in other orogenic belts. This work also emphasizes the need to distinguish between true mélange v. thrust sheets and broken formation in the field, and also cautions against use of the term ophiolitic mélange unless all the components of a dismembered ophiolite are actually present together.

New field, structural and geochronological data from the Shyok and Nubra valleys, northern Ladakh: linking Kohistan to Tibet

Abstract The Nubra-Shyok confluence in northern Ladakh is a key area for understanding the tectonic evolution of NW Himalaya and provides the basis for linking the geology of Pakistan to that of Tibet. The geology of the confluence area has been the subject of much speculation centred mainly on the existence of ophiolites and their regional significance. These ophiolites are thought to represent the eastward extension of the Shyok Suture Zone (SSZ), which separates the Dras island arc from the southern margin of Eurasia, and which was overprinted by movement along the Khalsar Thrust (often thought to represent the eastern continuation of the Main Karakoram Thrust). The geology of the area is relatively complex and the little information available has hampered regional geological correlations. The Khalsar Thrust (KT) and the dextral Karakoram Fault (KF), two regional tectonic features of NW Himalaya, merge at the confluence defining a triple point and three blocks: the Ladakh block to the south, the Saltoro block to the northwest, and the Karakoram block to the northeast. Close to the triple point, the KF changes strike and movement direction. Movement vector analysis of the triple point indicates that the KT and the two parts of the KF could have moved contemporaneously, and allows prediction of the movement vectors across the faults. The KT and KF shear preferentially volcano-sedimentary rocks of the Shyok and Nubra formations, respectively. Contrary to previous interpretations, these sheared rocks do not represent disrupted ophiolites. Regional tectonic reconstructions, however, require suturing between the Ladakh block and Eurasia and the strike of the SSZ in Baltistan suggests that the suture zone might crop out north of the KT, either along the southern slopes of the Saltoro Range or further north along the Saltoro valley. In the few outcrops of the Saltoro block we were able to visit, we found no evidence of ophiolitic rocks. Instead we found outcrops of the calc-alkaline Tirit batholith. Although our observations do not confirm the presence of the suture-related rocks in the southern Saltoro block, this possibility cannot be ruled out. Zircons from a sample of Tirit granite (U-Pb ion-microprobe age) yielded an age centred at 68 ± 1 Ma. The similar range of modal composition and age of the Tirit and Ladakh batholiths suggest that they are part of the same magmatic event. This result and a number of other observations indicate that the post-75 Ma geology of the Ladakh and Saltoro blocks is similar. Thus, if there is a suture zone in the southern Saltoro block, suturing must have occurred before 75 Ma, as concluded by others along the same tectonic boundary to the west in Pakistan. The KF represents a much younger terrane boundary, juxtaposing rocks of the Ladakh and Saltoro blocks to those of the Karakoram terrane. Rocks related to suturing of continents were not found along the KF. Karakoram leucogranites cropping out in the southern part of Karakoram terrane yielded a U-Pb zircon age centred at 15.0 ± 0.4 Ma (2σ). Because these leucogranites were not found south of the KF, this fault must have initiated after leucogranite intrusion and must therefore be younger than 15 Ma old. At the confluence the KF cuts across the regional rock sequence than can be followed from Kohistan into Baltistan and into the confluence area. Movement on the fault displaces the sequence by approximately 150 km to southeastern Tibet where the regional rock sequence can be regained.

Types and Distribution of Endogenic Gold Deposits in Western Sichuan, China

Endogenic gold deposits and prospects are widely distributed in western Sichuan, People's Republic of China. To the present time, 43 individual gold deposits and 2 associated gold deposits have been discovered, explored, and mined. All of these deposits belong to one of nine principal types, according to their host-rock series: (I) Archean high-grade metamorphic rock series–hosted; Phanerozoic intermediate-acid intrusive rock-hosted; (HI) ophiolitic melange rock series–hosted; (IV) carbonate rock series–hosted; (V) slightly metamorphic rock series–hosted; (VI) moderately metamorphic rock series–hosted; (VII) subvolcanic rock series–hosted; (VIII) slightly metamorphosed fine-grained clastic-subvolcanic rock series–hosted; and (IX) volcanic rock series–hosted. The tectonic setting, geological features, and temporal and spatial distribution of these different types of gold deposits and occurrences indicate that, to a great extent, gold mineralization reflects the evolution of tectonic environments in the western part of Sichuan Province throughout geological time.

Stable isotope geochemistry of Alpine ophiolites: a window to ocean-floor hydrothermal alteration and constraints on fluid-rock interaction during high-pressure metamorphism

The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however, despite its potential importance, fluid–rock interaction during high-pressure metamorphism is relatively poorly understood. The stable isotope and major element geochemistry of Pennine ophiolite rocks from Italy and Switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites, suggesting that they interacted with little pervasive fluid during high-pressure metamorphism. Cover sediments also have oxygen isotope ratios within the expected range of their protoliths. In the rocks that escaped late greenschist-facies retrogression, different styles of sub-ocean-floor alteration may be identified using oxygen isotopes, petrology, and major or trace element geochemistry. Within the basalts, zones that have undergone high- and low-temperature sub-ocean-floor alteration as well as relatively unaltered rocks can be distinguished. Serpentinites have δ18O and δ2H values that suggest that they were formed by hydration on or below the ocean floor. The development of high-pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub-ocean-floor alteration and which contained hydrated, fine-grained, reactive assemblages. Given that the transformation of blueschist-facies metabasic rocks to eclogite-facies assemblages involves the breakdown of hydrous minerals (e.g. lawsonite, zoisite, and glaucophane), and will thus liberate considerable volumes of fluids, metamorphic fluid flow must have been strongly channelled. High-pressure (quartz+calcite±omphacite±glaucophane±titanoclinohumite) veins that cut the ophiolite rocks represent one possible channel; however, stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids. Fluids were more likely channelled along faults and shear zones that were active during high-pressure metamorphism. Such strong fluid channelling may cause fluids to migrate toward the accretionary wedge, especially along the slab–mantle interface, which is probably a major shear zone. This may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting. Additionally, because fluids probably interact with relatively small volumes of rock in the channels, they cannot "scavenge" elements from the subducting slab efficiently.

Types and genesis of the enclaves in Central Anatolian granitoids

Central Anatolian granitoids range from tonalite–monzonite, through granodiorite, to alkali feldspar granite in composition, and typically have angular to oval-shaped mafic magmatic and metamorphic enclaves. These enclaves range from millimetres up to metres in size. The mineral compositions, textural features and amphibole chemistry reveal that the enclaves can be genetically differentiated into three types as xenolithic enclaves, magma segregation enclaves and magma mixing/mingling enclaves. The xenolithic enclaves are observed at the contact of granitoids with the metamorphic basement and/or ophiolitic rocks in Ortakoy (Akdagmadeni), Murmano and Akcakent plutons, and also in the southern part of the Agacoren Intrusive Suite. Based on their texture and mineral composition, these enclaves are identified as amphibolite, calc-schist and hornfels. The magma segregation enclaves, which are observed in almost all of the Central Anatolian granitoids, are monomineralic in composition and consist of accumulations of mafic minerals. They are small in size and best observed under the microscope. The magma mixing/mingling enclaves are the most abundant enclave type in Central Anatolian granitoids. They are microgranular in texture and rich in mafic minerals (mafic microgranular enclaves), and have characteristic petrographic features such as blade-shaped biotite, quartz ocelli, poikilitic feldspar and acicular apatite. Analyses of the amphiboles by electron micro-probe analysis reveal that the xenolithic enclaves have tremolitic–actinolitic hornblende compositions. The magma segregation and the magma mixing/mingling enclaves have mainly magnesio- and ferro-hornblende compositions, similar to those found in their host rocks. This similarity in the amphibole compositions suggests thermal and chemical equilibrium between the respective enclaves and their host attained during the course of their crystallization. Copyright © 1999 John Wiley & Sons, Ltd.

The Dabie-Sulu UHP rocks belt: review and prospect

The new results in the studies of the Dabie-Sulu UHP rocks belt during the past 5 years were summarized and discussed. The discussion included the following key points: ( i ) UHP eclogite has two kinds of country rocks, with one being UHP eclogite facies rocks and the other non-UHP granitic gneiss. ( ii ) The FeTiO3 in olivine indicated exsolution at depth of 300–400 km. However, the key point is to prove the peridotite in which the FeTlO3 in olivine was found once had been subducted down that depth. ( iii ) UHP hydrous phase evidenced that fluids had taken part in the UHP metamorphism, while the meter-scale inhomogeneous distribution of O-, C-isotope indicated no fluid activity in the deep subduction environment. ( IV ) No agreement has been arrived on many problems related to the tectonic background of the UHP rocks, such as “whether or not ophiolitic rocks there exist now?”, “when did UHP metamorphism proceed?”, “what is the subdution polarity?”, etc. ( V ) How did the UHP rocks exhume from mantle depth? The future studies will focus on the following three subjects: ( i ) thermal dynamics of the UHP metamorphism, ( ii ) relationship between UHP metamorphism and collision orogeny, as well as their geodynamics, and ( iii ) interactions between crust and mantle, and between continental lithosphere and asthenosphere during the collision orogenic process, as well as their constraints to the evolution of continental lithosphere.

Sedimentological characteristics and morphology of the aeolian sand dunes in the eastern part of the UAE, a case study from Ar Rub' Al Khali

The Umm Az Zimul–Al Wijan area is located in the southeastern part of the UAE and covers about 1600 km 2. It is bounded to the east by the Oman Mountains and to the south and west by the extensive dune area of Ar Rub' Al Khali. Field investigation revealed the common occurrence of parallel to sub-parallel sand ridges with a general east–west orientation. Also present, but less frequent, are barchan, barchanoid and star dunes. Interdune areas are classified into four types that include: dolomite, mixed ancient sabkha and ophiolitic rock fragments, mixed sand sheet and ophiolitic rock fragments, and recent sabkha. The textural characteristics of the sand forming the dunes are unimodal with a modal class in fine sand size, moderately to very well sorted, positively skewed and mesokurtic. Quartz grains are mainly subrounded to subangular. The similarity in roundness properties in different dune types in the eastern province of the UAE seems not to be influenced by dune form. Based on the heavy mineral suites and the angularity of quartz grains, it is believed that the sand sediments are mostly derived from the ultrabasic rocks forming the ophiolitic sequence of the Oman Mountains, acidic rocks of Iran and from the coastal area.

ORIGIN OF HIGHLY REFRACTORY PERIDOTITES: IMPLICATIONS FOR DIVERSITY OF HIGH-MG ANDESITE MAGMA GENESIS

Highly refractory peridotites are defined here as peridotites more depleted in melt components than abyssal peridotite, i.e. peridotites with spinel showing Cr# (Cr/(Cr+Al) atomic ratio) higher than 0.6. These correspond to the type III peridotite defined by Dick and Bullen (1984) and are reported from forearc regions, such as Izu-Ogasawara, Mariana and Tonga trenches (e.g. Bloomer and Hawkins, 1983; Bloomer and Fisher, 1987; Ishii et al., 1992) and from the Kamuikotan belt, the central axial zone of Hokkaido, northern Japan (e.g. Katoh and Nakagawa, 1986; Makita and Arai, 1997). These peridotites are, however, not common in the upper mantle and their genesis has not been explained thoroughly. We discuss the genesis of highly refractory peridotite from three complexes, the Takadomari, Iwanaidake and Nukabira, in the Kamuikotan belt, Japan, and its bearing on the process of high-Mg andesite magma genesis. The Kamuikotan belt is a tectonic melange zone composed of metamorphic rocks, ultramafic rocks, greenstones and sedimentary rocks. The metamorphic rocks are of typical high-pressure/low-temperature type and ophiolitic rocks which recorded low-pressure ocean-floor metamorphism are distributed in the Horokanai area, northern part of the belt (Ishizuka et al., 1983). Ultramafic rocks form the basal member of the ophiolite in the northern part and are exposed as peridotite complexes of various size in the southern part of the Kamuikotan belt. They are generally in fault contact with surrounding Kamuikotan metamorphic rocks, sediments and mafic volcanic rocks (e.g. Niida and Katoh, 1978). The peridotite complexes mainly consist of harzburgite and dunite which suffered serpentinization to various extent. The Takadomari complex from the northern part, and the Iwanai-dake complex from the southern part of the belt consist of harzburgite, dunite and small amounts of orthopyroxenite. The Nukabira complex (southern part of the belt) consists of lherzolite and harzburgite with dunite and pyroxenites. Podiform chromitite deposits sometimes accompany dunite in the latter two complexes. Primary hydrous minerals are sometimes included in chromian spinel of the former two complexes. The Cr# of chromian spinel and Fo (forsterite) content of olivine in peridotites from the Takadomari complex are high (Cr#=0.64-0.92, Fo=91.9- 94.0), and are higher in dunite than in harzburgite on average. The Cr# and Fo are also high (Cr#=0.43-0.87, Fo=90.8-93.5) in the Iwanai-dake complex and have wider ranges (Cr#=0.18-0.86, Fo=89.0-93.2) in the Nukabira complex. The Cr# of spinel and Fo of olivine in dunite complexes are sometimes similar to or even lower than the values in harzburgite from Iwanai-dake and Nukabira complexes. The mineral chemistry of dunites in the Iwanai-dake and Nukabira complexes depends on their thickness within harzburgite or lherzolite bodies. Thicker dunite layers tend to show higher Cr# of chromian spinel and Fo content of olivine. In addition to this, there are clear systematic variations in lithology across dunite layers and surrounding peridotites. In the case of thick dunite layers, the Cr# of chromian spinel and the Fo content of olivine gradually increase from the surrounding peridotite to the central part of the dunite layer (Cr#>0.6 and Fo>91.5 in dunite). In contrast to this fact, these values in thin dunite layers are similar to the surrounding peridotite, lherzolite or harzburgite (Cr#=0.4- 0.6 and Fo=89-91). These values are generally lower in thinner dunite layers than in the thick ones compared at the center. The harzburgites and dunites in the Takadomari complex are interpreted to be a series of refractory residue after extraction of high-Mg andesite magma; partial melting of the fertile peridotite possibly occurred under hydrous conditions. Harzburgite and lherzolite are mostly simple residue in the Iwanai-dake and Nukabira complexes but their dunites are not simple residue because the Cr# and Fo content are not systematically higher in the dunite than in the harzburgite (e.g. Arai, 1987; 1994). The dunite may be produced by a reaction between wall peridotite and a melt formed at higher pressures (e.g. Quick, 1981; Fisk, 1986; Kelemen, 1990). The differences in thickness and mineral chemistry of dunite layers are due to the difference of the melt/wall peridotite volume ratio. Supplying a large amount of melt caused formation of thick dunite layers and generated high-Mg andesitic magma by the reaction process.

GEOCHEMISTRY, PETROLOGY AND THE ORIGIN OF SHAHR-BABAK OPHIOLITE, CENTRAL IRAN

The Iranian ophiolites have been divided into four groups (Takin, 1972; Stocklin, 1974; McCall, 1997; Ghazi and Hassanipak, 1999): i) ophiolites of northern Iran along the Alborz range; ii) ophiolites of the Zagros Suture Zone, including the Neyriz and the Kermanshah ophiolites, which appear to be extensions of the Oman ophiolite emplaced onto the Arabian continental mass; iii) ophiolites and coloured melanges of the Makran region which are located to the south of the Sanandaj-Sirjan microcontinental block, including unfragmented complexes such as Sorkhband and Rudan, and iv) ophiolites and coloured melanges that mark the boundaries of the internal Iranian microcontinental block, including some of those in the Makran region (e.g., Band-e-Zeyarat, Dar Anar, Ganj) and those inside of the Sanandaj-Sirjan microcontinental block (Shahr-Babak, Nain, Baft, Sabsevar and Tchehel Kureh). The Shahr-Babak ophiolite is a highly dismembered ophiolite complex located along the Nain-Baft fault zone, which marks the western border of the Central Iranian microcontinent. The Shahr-Babak ophiolite is considered to be one complexes of the internal Iranian group of ophiolites and coloured melanges (Fig. 1). The igneous rocks of this complex consist of both mantle and crustal suites. The mantle rocks include peridotites (lherzolites, harzburgites and dunites), serpentinites and webstrite-pyroxenites. The crustal rocks include both plutonic and volcanic sequences. The plutonic rocks consist of gabbros, diorites, and plagiogranites. The volcanic sequence exhibits a wide range in composition from basaltic andesite to rhyodacites-rhyolites and trachyandesites. Also present are extensive units of radiolarian chert which are interbedded within the basaltic andesites. A combination of petrographic observations and analyses of incompatible trace elements and rare earth elements (REE) indicates the presence of at least three different types of extrusive rocks in the Shahr-Babak ophiolite (Fig. 2). The chemical data on these extrusive rocks clearly shows that the extrusive rocks were formed from two distinct magmatic sources; i) the melts that produced the basaltic andesites and rhyodacites were generated in an island arc type environment, and ii) the melt that produced the trachyandesites was generated in a within-plate environment (e.g. intraplate-oceanic island). The geochemical data suggest that the basaltic andesite melt was generated either by a large degree of partial melting (ca. 40%) of a mantle lherzolite source or by a lesser degree of partial melting (15-20%) of a previously depleted mantle source. The data also suggests that the gabbros, diorites, plagiogranites, basaltic andesites and rhyodacite-rhyolites are petrogenetically related and were produced by fractionation controlled by removal of plagioclase and to a lesser extent clinopyroxene. The presence of compositionally diverse extrusive rocks (basaltic andesite, andesites, rhyodacite, rhyolites and trachyandesites) intercalated with a variety of Triassic-Cretaceous sedimentary rocks indicates that the Shahr-Babak ophiolite has both island arc and intra-oceanic components. These components were brought together during northeast dipping subduction of a narrow belt of oceanic lithosphere under the Central Iranian microcontinent and Eurasia. This seaway was part of the southern Tethyan ocean which surrounded the Central Iranian microcontinent.

THE MONTGENEVRE OPHIOLITE MANTLE SECTION. FROM GENESIS TO LATER PROCESSES: PETROLOGICAL AND STRUCTURAL CONSTRAINTS

The Alpine Piedmont Ligurian ophiolites are considered to be remnants of the Jurassic Tethyan ocean. Numerous similarities with northern Apennines ophiolites have been emphasized, particularly with the external belt. The most typical features of these ophiolites are: (1) a lherzolitic mantle section, (2) a reduced cumulate high level-type gabbroic portion, without well defined layering, (3) a lack of a sheeted- dyke complexes; (4) a reduced and discontinuous MORB crustal section. An upwelling of lherzolitic mantle with a low degree of partial melting, in a slow-spreading ridge environment, is generally proposed for the genesis of such a succession. The minor proportion of gabbros together with their high-level type characteristics are considered to be the result of crystallization in small magma chambers dispersed within the lherzolitic upper mantle. Except for minor exceptions, primary contacts between upper mantle and gabbro sections are not observed. This is due to modifications during oceanic and alpine tectonism. The Montgenevre ophiolite complex (MOC) is well known for having largely escaped Alpine metamorphic overprinting. Comprising several tectono-lithological units, it presents a complete pseudo-stratigraphy. Thus it is a favourable context for the study of primary features and oceanic transformations. Peridotitic rock-types are intensely serpentinized (> 90% most often, locally up to 70% only). Mantle peridotites are mainly lherzolites, but minor associated wehrlitic, harzburgitic, and dunitic zones are associated. Plagioclase-bearing peridotites are common. Serpentinites crop out either: (1) in the basal complex of the MOC, which is characterized by ophiolitic masses, and detritus, incorporated within the supra-ophiolite sedimentary cover and the schistes lustres, or, (2) mainly, in the upper tectonic unit, or klippe, essentially composed of the ophiolitic succession. The serpentinized peridotites are dominantly exposed at the eastern part of the upper unit. At the western part, with dominant submarine volcanites, serpentinites, often crushed, are localized only at the contact zone with the basal complex or along the contacts between lithological units, particularly between gabbro and serpentinized peridotite, testifying for a tectonic relationship. Several primary characteristics are still recognizable within the serpentinites: (1) type of original peridotite, (2) impregnation or not of basaltic liquid, (3) mantle tectonite or ultrabasic cumulate textures, (4) reworked serpentinites sometimes capped by a supra-ophiolite sedimentary cover. Moreover, as classically observed, pyroxenitic and troctolitc- gabbroic levels, gabbro and dolerite more or less rodingitic dykes, and albitite are associated within the mantle section. Petrological and structural considerations are obtained from the study of such relationships. The macroscopic and microscopic structures observed within the Montgenevre ophiolite upper mantle ultrabasic rocks, in a context apparently close to the petrological Moho, are mostly of tectonite type with high temperature plastic deformation of olivine and orthopyroxene, typical of asthenospheric conditions. Sometimes mylonitization developed in the oceanic environment, certainly during transverse or longitudinal fracturing of the lithosphere. The latter observation is typical of slow spreading ridge environments as observed for instance in the MAR Vema fracture zone. Magmatic liquid percolation locally modified the peridotitic rocks (e.g., impregnated lherzolites) and can, in some cases, when very close to the Moho, produce troctolitic lithologies belonging to the lowermost part of the cumulate section. Such rocks are sometimes difficult to differentiate from mantle peridotitic types exhibiting major melt impregnation, or even more or less diffuse melt “pockets”, and also caracterized by cumulate textures. There is no difference in primary mineralogical composition or in textural features, but the troctolitic rocks in the mantle suffered from rodingitic recrystallizations, whereas they are saussuritized in the lowermost crustal section. Another discriminant way is structural, based on lattice study of olivine crystals which should be undeformed (or very slighly plastically deformed) with low concentration of intracrystalline defects when related to the lowermost cumulate crustal section. For comparable mantle related ultrabasic cumulates, olivine is indeed strongly plastically deformed (with high intracrystalline defects concentration). Other presented petrological considerations are mainly based on geochemical data (major, trace and rare earth elements, and lead isotopes analyses).

Geochemistry of the ophiolite and island-Arc volcanic rocks in the Mianxian-Lueyang suture zone, southern Qinling and their tectonic significance

Ultrabasic rocks in the Mianxian-Lueyang ophiolitic melange zone include harzburgite and dunite which exhibit LREE depletion with remarkable positive Eu anomaly. The diabase dike swarm shows LREE enrichment but slightly negative Eu anomaly. Metamorphosed volcanic rocks can be divided into two groups in terms of their REE geochemistry and trace element ratios of Ti/V, Th/Ta, Th/Yb and Ta/Yb. One is the MORB-type basalt with LREE depletion, representing the fragments of oceanic crust and implying an association of the MORB-type ophiolite and an ancient ocean basin between theQinling and Yangtze plates during the Middle Paleozoic-Early Mesozoic era. The other comprises the island-arc volcanic rocks including tholeiitic basalt and a large amount of calc-alkaline intermediate-acid volcanic rock, which could not be the component of the ancient oceanic crust but the result of magmatism at the continental margin. This indicates that the Mianxian-Lueyang limited ocean basin had undergone a whole process of development, evolution and vanishing from Devonian-Cretaceous to Permian. And the Qinling area had become an independent lithospheric microplate, on the southern side of which there were exhibited the tectonic characteristics of active continental margins during the Late Paleozoic-Early Mesozoic. That is to say, the Qinling cannot be simply considered as a result of collision between the Yangtze and North China plates.

Structure and Collision History of the Buton Continental Fragment, Eastern Indonesia

Although commonly reservoired within Tertiary rocks, hydrocarbons in eastern Indonesia have generally been sourced from Mesozoic sediments deposited on the continental margin of northern Australasia. Fragments of this margin are now widely dispersed as allochthonous terranes throughout the area, one of the most far traveled examples being the island of Buton, southeast of Sulawesi. Asphalt reserves on Buton support a significant local industry and exploration continues for oil and natural gas. In common with other prospective Australian-derived terranes around the margins of the Banda Sea, Buton is now separated from Australia by the active plate boundary marked by the Java Trench and the collision trace along the Timor, Tanimbar, and Seram troughs. Buton differs from these other terranes in its distance from that boundary and its consequent insulation from the effects of the Pliocene-Pleistocene collision between Australia and the Banda arcs. Reconstruction of the geological history of the Buton terrane thus has an important role in guiding future exploration in the other Australasian fragments in eastern Indonesia. Geophysical studies of the Buton region have used seismic reflection, gravity, and magnetic (including paleomagnetic) techniques. Seismic reflection images generally record extension rather than compression as dominating the recent history of the area. Gravity data define the present-day western limits of the Buton terrane and suggest that in the east the terrane includes the almost entirely submerged Tukang Besi platform. The gravity surveys also demonstrate that the ophiolitic rocks exposed on Buton are not attached to deep roots, but are thin and isolated overthrust sheets. Therefore they do not mark a terrane boundary and their presence has little bearing on the prospectivity of the area. Paleomagnetic results document the independent movements of thrust sheets on Buton during the Pliocene-Pleistocene. The combined data from Buton record its separation from Australia as part of a microcontinental block in the Jurassic or Late Triassic, followed by collision with the Eurasian margin in southeastern Sulawesi in the Oligocene or early Miocene. Collision was followed by extension (as in Sulawesi itself) producing minor separation of Tukang Besi from Buton and much greater dispersion of other fragments of the microcontinent, some of which have since been incorporated in the new collision zone in the Outer Banda arc. The oil seeps and asphalt deposits of Buton are proof that hydrocarbons in the Banda arc fragments can be sourced from within these fragments and are not necessarily derived from the underthrusting Australian margin.

Site attenuation and source parameters on the North Anatolian Fault zone, eastern Turkey estimated from the aftershocks of 13 March 1992 Erzincan earthquake

Site attenuation and source characteristics of 45 aftershocks of the 13 March 1992 Erzincan, eastern Turkey earthquake have been determined from SH-wave spectra using a least-squares best-fit method. Although the most of the seismograms were recorded on the ophiolitic rock sites and the average regional attenuation correction of Q(f) = 35 f0.83 was applied, the high fall-off rates γ ranging between 3.0 and 4.5 beyond the corner frequencies of 8–13 Hz were observed on the SH spectra. The site attenuation value, κ, has been calculated from the slope of the high frequency part of the SH spectra. It was found that κ varied in the range of 0.0124–0.0364 s and the average was 0.0246 s with a standard deviation of 0.0047 s. The high fall-off rates of observed spectra have decreased considerably as a result of site attenuation corrections and converged to an average value of 2.3. We concluded that the high fall-off rates of SH wave spectra are mainly controlled by highly deformed Miocene ophiolitic formations which covers a wide area in the North Anatolian Fault zone in the vicinity of Erzincan region. Using the spectra that were corrected for regional and site attenuation, and assuming a Brune's source model; seismic moment, source radius, and stress-drops of the aftershocks were computed. We found that stress-drops for some of the aftershocks in Erzincan area have slightly decreased after removal of site attenuation over SH wave observed spectra.