Quartzofeldspathic Schists Research Articles

Detrital zircon U–Pb and Hf analyses of Silurian–Devonian sediments in the Sukhothai Terrane, northern Thailand: implications for the Mid-Paleozoic arc belt

The Sukhothai Terrane in northern Thailand consists of continental basement and a Permo-Triassic magmatic arc related to the subduction of the main Palaeo-Tethys Ocean. The Donchai Group represents the oldest sedimentary sequence of the Sukhothai Terrane and consists mainly of meta-sandstone, quartzo-feldspathic schist, phyllite and silty slate. We present new detrital zircon U–Pb geochronology, Lu–Hf isotope data and geochemical data for the sedimentary rocks of the Donchai Group to elucidate the depositional age, provenance and nature of the group. The youngest detrital zircon ages (433–403 Ma) suggest that the Donchai Group accumulated between 433 and 368 Ma, revealing Silurian–Devonian arc magmatic rocks on the western margin of the Sukhothai Terrane. Sediments of the Donchai Group were sourced from both the continental basement and a Silurian–Early Devonian magmatic arc, suggesting a depositional setting on the continental slope of a back-arc basin along the western flank of the Sukhothai Terrane. The Silurian–Devonian arc belt in SW China probably extends to the Chiang Rai region to the west of the Sukhothai Terrane, northern Thailand, revealing the northwards subduction of the Proto-Tethys Ocean along the western Simao and Sukhothai margin during the mid-Paleozoic. The inferred arc and back-arc configuration of the Proto-Tethys Ocean in northern Thailand is comparable with that recently established in Yunnan, SW China. Supplementary material: Detrital zircon U–Pb ages and trace element analyses, zircon in-situ Lu–Hf isotopic compositions and whole-rock geochemical data are available at https://doi.org/10.6084/m9.figshare.c.6858373

PETROGRAPHY AND GEOCHEMISTRY OF METASEDIMENTS FROM BUGAJI, PART OF MALUMFASHI SHEET 79 NE, KANO STATE, NORTHWESTERN NIGERIA

This research is aimed at determining the protolith of metasediments that underpin the geology of Bugaji region of Kano State. The study area was mapped systematically and the lithologies encountered include phyllite, undifferentiated schist, quartzo-feldspathic schist, and gneiss. Samples taken from several outcrops were subjected to mineralogical and geochemical analyses using X-ray diffraction and X-ray Fluorescence (XRF), respectively. The metasedimentary rocks have evidently undergone significant deformation textures (foliation, lineation, and fractures), and metamorphism, indicating the area is situated in a tectonically active zone. Quartz, feldspar, mica, and a number of accessory minerals make up majority of the metasediments, while the geochemical data show enrichment of aluminum, silica, and iron. The geochemical indicators support the idea that the rocks developed on a continental margin. The mineralogy and geochemistry indicate the rocks likely came from a source region with a predominance of granitic rocks.

Cretaceous molybdenite in metasomatic epidosite associated with the Pounamu ophiolite, New Zealand

ABSTRACTEpidosites occur in association with pelagic and quartzofeldspathic metasediments overlying the Pounamu suprasubduction zone ophiolite. They are dominated by epidote with minor diopside, tremolite, titanite, zircon and quartz. Detrital zircon age spectra are similar to those of associated quartzofeldspathic schist, with minimum ages of Cretaceous (c. 106 Ma). Epidosite was derived by metasomatic replacement of this metasedimentary protolith by Ca-rich fluids, likely derived from the underlying ophiolite. One epidosite contains laminae and veinlets of molybdenite, which yields a Re/Os age of c. 89.7 ± 0.6 Ma. Metasomatic formation of epidosite and molybdenite mineralisation occurred during the interval between c. 106 and c. 70 Ma when the Pounamu terrane component of the Alpine Schist underwent accretion, subduction, hydrothermal alteration, mineralisation and metamorphic recrystallization. Molybdenite occurrence is unique in the context of Haast Schist metallogenesis, with Mo most likely derived by leaching and redistribution from the sedimentary cover of the Pounamu ophiolite.

The internal structure and composition of a plate-boundary-scale serpentinite shear zone: the Livingstone Fault, New Zealand

Abstract. Deciphering the internal structure and composition of large serpentinite-dominated shear zones will lead to an improved understanding of the rheology of the lithosphere in a range of tectonic settings. The Livingstone Fault in New Zealand is a terrane-bounding structure that separates the basal portions (peridotite; serpentinised peridotite; metagabbros) of the Dun Mountain Ophiolite Belt from the quartzofeldspathic schists of the Caples and Aspiring Terrane. Field and microstructural observations from 11 localities along a strike length of ca. 140 km show that the Livingstone Fault is a steeply dipping, serpentinite-dominated shear zone tens of metres to several hundred metres wide. The bulk shear zone has a pervasive scaly fabric that wraps around fractured and faulted pods of massive serpentinite, rodingite and partially metasomatised quartzofeldspathic schist up to a few tens of metres long. S–C fabrics and lineations in the shear zone consistently indicate a steep east-side-up shear sense, with significant local dispersion in kinematics where the shear zone fabrics wrap around pods. The scaly fabric is dominated (>98 % vol) by fine-grained (≪10 µm) fibrous chrysotile and lizardite–polygonal serpentine, but infrequent (<1 % vol) lenticular relicts of antigorite are also preserved. Dissolution seams and foliation surfaces enriched in magnetite, as well as the widespread growth of fibrous chrysotile in veins and around porphyroclasts, suggest that bulk shear zone deformation involved pressure–solution. Syn-kinematic metasomatic reactions occurred along all boundaries between serpentinite, schist and rodingite, forming multigenerational networks of nephritic tremolite veins that are interpreted to have caused reaction hardening within metasomatised portions of the shear zone. We propose a conceptual model for plate-boundary-scale serpentinite shear zones which involves bulk-distributed deformation by pressure–solution creep, accompanied by a range of physical (e.g. faulting in pods and wall rocks; smearing of magnetite along fault surfaces) or chemical (e.g. metasomatism) processes that result in localised brittle deformation within creeping shear zone segments.

Details Geological Mapping and Petrological Characteristics of EL Shereik Study Area, River Nile State, North Sudan

The present study focused on details geology of the lithostratigraphy, phenomena and petrogenesis, with full classification of rock units and correlate by the regional geology in the study area. The previous studies, focused on regional geology, which can be divided in Bayuda Terrane, represented by Dem El Tor Shear Zone (DTSZ), its Precambrian to Tertiary ages (isotopic signatures of later cooling 590 to 550 Ma), with dominated by metamorphic and intrusive rocks belonging to the Pre-Nubian basement complex. The study area lies at El Shereik, River Nile State, Sudan. It is characterized by low-lying bed plains, covered with superficial deposits, with an arid climate. The techniques of this work, represented in an official, field works and laboratory test, with used Land Sat Images ETM. And Nubian Arabian Shield, represented by the Keraf Shear Zone (KSZ), its Precambrian to Phanerozoic ages (Neoproterozoic, Pb isotope ages dating, of forms is -730 -710Ma and ended in -565Ma), with dominated by metamorphic, ophiolitic mélange, volcano-sedimentary sequences, and Phanerozoic sediments. But through this detailed surveyed, we discovered different types of rock units; they were found as accumulated and highly deformed, affected by various thrusting faults. collected more than 40 samples, through six traverses, for classification and petrographic studies and classified more than 9 types of rooks units didn’t mentioned before in previous study, all of them well exposed as following: metamorphic rocks, included, migmatites, calc-silicates, wollastonite, talc-schist, carbonaceous (dolomitic marble), amphibolites, graphitic schist, mica schist, quartzo-feldspathic schist, and grey gneiss, While the Igneous rocks, consisted of dykes, as dolerite, trachy-basaltic, Rhyolite porphyry, pegmatites, diorite, and quartz veins, whereas the superficial deposits, included, of aeolian, fluvial, and collovial. These rocks extend and spread to the outside of the limits of the study area and most of them oriented parallel with KSZ SE - NW region and a few of them are oriented E-W, This is maybe due to the collusion of the contact boundary between KSZ and DTSZ Bayuda Tehran. Talc-schist, Wollastonite and graphitic schist, represent a strategic stockpile besides gold mining. Studies conducted with DTSZ, its old age, occurred before Neoproterozoic compared to KSZ. Because it has the first deformation of the folding of pre- KSZ proportion to the presence of folding in the west and east of the Nile zone, according to the border between Bayuda Terrane and Nubian Arabian Shield, as the suggest result. And the previous studies of the ages dating confirm it.

Oxy-dravite from Wołowa Góra Mountain, Karkonosze massif, SW Poland: Crystallochemical and structural studies

ABSTRACTYellowish dravitic tourmaline (dominated by the oxy-dravite component) associated with secondary fluor-dravite/fluor-schorl and dravite/schorl tourmalines was found in a quartz vein cropping out in the eastern part of the Karkonosze Mountains range, SW Poland. The crystal structure of this tourmaline was refined to an R1 value of 1.85% based on single-crystal data, and the chemical composition was determined by electron-microprobe analysis. The tourmaline, a representative of the alkali-tourmaline group, has the structural formula: (Na0.75Ca0.12□0.13)Σ1(Mg1.93Al0.95Ti0.06${\rm Fe}_{{\rm 0}{\rm. 04}}^{{\rm 2 +}} $ V0.01)Σ3(Al5.38Mg0.62)Σ6B3Si6O27(OH)3(O0.46OH0.33F0.21)Σ1, and is characterized by an extremely high Mg/(Mg + Fe) ratio of 0.97–0.99, the WO2– content that reaches 0.59 apfu resulting in a local predominance of the oxy-dravitic component and Mg–Al disorder on the octahedral Y and Z sites of the order of 0.64 apfu. This disordering results in an increasing <Z–O> distance with ~1.925 Å, and unit-cell parameters a = 15.916(1) Å and c = 7.180(1) Å. The tourmaline formed during Variscan prograde metamorphism under the influence of a released (H2O,B,F)-bearing fluid. The fluid mobilized the most soluble components of partly altered silicic volcaniclastic material of the Late Cambrian to Early Ordovician bimodal volcanism to become the protolith for adjacent quartzo-feldspathic schists and amphibolites, and propagated them into the surrounding granitic gneisses of the Kowary unit in the eastern metamorphic cover of the Karkonosze granite.

Timing and duration of regional metamorphism in the Kagasawa and Unazuki areas, Hida metamorphic complex, southwest Japan

U–Pb zircon geochronology of two granite intrusions containing bluish‑tint K-feldspars and two biotite-hornblende gneisses in the Kagasawa area, situated at the center part of the Hida metamorphic complex, southwest Japan, reveals the early Triassic low P/T metamorphism in the Hida metamorphic complex (251–247Ma). The granite intrusions into the Hida gneiss occurred at two different stages (235.4±1.6Ma and 183.6±1.8Ma) and thermally affected the Hida gneiss. In the Unazuki area, situated at the northeast margin of the Hida metamorphic complex, a quartzo-feldspathic schist and a biotite granite containing a quartzo-feldspathic schist as xenolith yielded U–Pb zircon ages of 258.0±2.3Ma and 253.0±1.9Ma, respectively, which indicates that the medium P/T metamorphism in the Unazuki area occurred in late Permian. The similarity between the geochemical compositions in whole rocks and zircons of the quartzo-feldspathic schist and the granite suggests that felsic volcanics were deposited on basement rocks at 258Ma and, after affecting the medium P/T metamorphism, residual felsic magma was exhumed while taking the Unazuki schists as a xenolith at 253Ma. Although the time interval between protolith formation and post-peak regional metamorphism in the Unazuki area (0.8–9.2Ma) is similar to that in the Kagasawa area (1–8.6Ma), the different timing of the regional metamorphism indicates that the protolith of the Unazuki metasediments was deposited not on the proximal Hida metamorphic complex but on ancient rocks containing Eoarchean-Paleoproterozoic components of the North China Craton.

Reactivation of normal faults as high-angle reverse faults due to low frictional strength: Experimental data from the Moonlight Fault Zone, New Zealand

Large normal faults are frequently reactivated as high-angle reverse faults during basin inversion. Elevated fluid pressure is commonly invoked to explain high-angle reverse slip. Analogue and numerical modeling have demonstrated that frictional weakening may also promote high-angle reverse slip, but there are currently no frictional strength measurements available for fault rocks collected from large high-angle reverse faults. To test the hypothesis that frictional weakening could facilitate high-angle reverse slip, we performed single- and double-direct friction experiments on fault rocks collected from the Moonlight Fault Zone in New Zealand, a basin-bounding normal fault zone that was reactivated as a high-angle reverse fault (present-day dip angle 60°–75°). The fault core is exposed in quartzofeldspathic schists exhumed from c. 4–8 km depth and contains a <20 m thick sequence of breccias, cataclasites and foliated cataclasites that are enriched in chlorite and muscovite. Friction experiments on water-saturated, intact samples of foliated cataclasite at room temperature and normal stresses up to 75 MPa yielded friction coefficients of 0.19<μ < 0.25. On the assumption of horizontal maximum compressive stress, reactivation analysis indicates that a friction coefficient of <0.25 will permit slip on high-angle reverse faults at hydrostatic (or even sub-hydrostatic) fluid pressures. Since foliated and phyllosilicate-rich fault rocks are common in large reactivated fault zones at basement depths, long-term frictional weakening is likely to act in concert with episodic build-ups of fluid pressure to promote high-angle reverse slip during basin inversion.

Fault-zone structure and weakening processes in basin-scale reverse faults: The Moonlight Fault Zone, South Island, New Zealand

The >200 km long Moonlight Fault Zone (MFZ) in southern New Zealand was an Oligocene basin-bounding normal fault zone that reactivated in the Miocene as a high-angle reverse fault (present dip angle 65°–75°). Regional exhumation in the last c. 5 Ma has resulted in deep exposures of the MFZ that present an opportunity to study the structure and deformation processes that were active in a basin-scale reverse fault at basement depths. Syn-rift sediments are preserved only as thin fault-bound slivers. The hanging wall and footwall of the MFZ are mainly greenschist facies quartzofeldspathic schists that have a steeply-dipping (55°–75°) foliation subparallel to the main fault trace. In more fissile lithologies (e.g. greyschists), hanging-wall deformation occurred by the development of foliation-parallel breccia layers up to a few centimetres thick. Greyschists in the footwall deformed mainly by folding and formation of tabular, foliation-parallel breccias up to 1 m wide. Where the hanging-wall contains more competent lithologies (e.g. greenschist facies metabasite) it is laced with networks of pseudotachylyte that formed parallel to the host rock foliation in a damage zone extending up to 500 m from the main fault trace. The fault core contains an up to 20 m thick sequence of breccias, cataclasites and foliated cataclasites preserving evidence for the progressive development of interconnected networks of (partly authigenic) chlorite and muscovite. Deformation in the fault core occurred by cataclasis of quartz and albite, frictional sliding of chlorite and muscovite grains, and dissolution-precipitation. Combined with published friction and permeability data, our observations suggest that: 1) host rock lithology and anisotropy were the primary controls on the structure of the MFZ at basement depths and 2) high-angle reverse slip was facilitated by the low frictional strength of fault core materials. Restriction of pseudotachylyte networks to the hanging-wall of the MFZ further suggests that the wide, phyllosilicate-rich fault core acted as an efficient hydrological barrier, resulting in a relatively hydrous footwall and fault core but a relatively dry hanging-wall.

The P–T path of eclogites in the St. Cyr klippe, Yukon, Canada: Permian metamorphism of a coherent high-pressure unit in an accreted terrane of the North American Cordillera

The St. Cyr klippe, part of the Yukon–Tanana terrane in Yukon, Canada, preserves fresh and retrogressed eclogite hosted by quartzofeldspathic schists. Petrology, mineral chemistry and compositional zoning combined with isochemical phase equilibrium (pseudosection) modeling shows that eclogites followed a clockwise pressure–temperature path. An amphibolite facies, pre-eclogite stage (pre-Stage I) preceded early eclogite-facies conditions of about 16 kbar and 510 °C (Stage I) and the peak pressure (Stage II), which reached up to 20 kbar and 670 °C. The peak observed and modeled eclogite-facies assemblage is garnet, omphacite, Na-rich amphibole, phengite, quartz and rutile. Pseudosections were modeled for varying Fe 3+ contents, and Fe 3+ = 20 % of total Fe is found to best match the pyrope and grossular content observed in garnet. Retrogression back to amphibolite-facies conditions (Stage III) followed a clockwise path as temperature decreased during decompression without passing through the chlorite stability field. U-Pb SIMS (secondary ion mass spectrometry) dating and trace-element analysis of zircon shows that the protolith of the eclogites formed within the Yukon–Tanana terrane during early arc activity, between 364 and 380 Ma. The rocks were then subducted to mantle depths and metamorphosed to eclogite-facies conditions during the Late Permian, between 267 and 271 Ma. These medium-temperature eclogites are part of a coherent high-pressure (HP) terrane that was tectonically eroded from the Yukon-Tanana composite arc in a subduction zone environment.

Gold Mineralisation in Chigargunta Area of the Kolar Schist Belt, South India- A Part of the Archean Greenstone Belt

Gold mineralisation is reported for the first time in ‘Champion gneiss’ (quartzo-feldspathic schist) a felsic unit,in the eastern sector of the Chigargunta area (Lat: 120 430300N, Long:780 150 000E) of the Kolar schist belt, South India, during 1979-80. Quartzo-feldspathic gneiss and hornblende schist are the predominant rock types of the area. These rocks are characterised by a strong pervasive foliation trending N50E –S50W to N200E - S200W with easterly dips of 70-85. In the northern part of the area the rocks are folded with the foliation which is axial planar. Lower to middle amphibolite facies metamorphism is widespread in the area. Gold mineralisation is localised along shear zones which are ductile to brittle in nature. These zones are parallel to subparallel to the trend of foliation in the host rock and are characterized by strong mylonitic fabric, profuse quartz venation and hydrothermal alteration. Pyrite and pyrrhotite are the dominant sulphides. Gold occurs in native form. This felsic hosted gold mineralisation termed as E-2 lode (for exploration purpose) is estimated to contain a reserve of 3.13 million tonnes averaging 4.7 g/t gold. Besides, this lode there are several loads ( E-1,E-3, etc) occur within mafic units as well as in felsic unit adjacent to E-2 lode with different reserves, within the schist belt. Epigenetic gold mineralisation shows a close temporal and spatial relationship to late Archean (2700-2500 m.y) crustal accretion, stabilisation and granulite formation in the South Indian Shield. The present work delineate, the nature of mineralization in felsic unitwithin the schist belt which was not considered as source rock of gold mineralization during the period and left untouched until the present work

Detrital zircon U–Pb–Hf and O isotope character of the Cahill Formation and Nourlangie Schist, Pine Creek Orogen: Implications for the tectonic correlation and evolution of the North Australian Craton

Detrital zircon age and Hf isotope patterns for the Cahill Formation and Nourlangie Schist are distinctly different from other Paleoproterozoic successions in the North Australian Craton. The Cahill Formation and Nourlangie Schist comprise the bulk of the Paleoproterozoic strata in the Nimbuwah Domain, the easternmost part of the Pine Creek Orogen on the northern margin of the North Australian Craton. They comprise micaceous and quartzofeldspathic schist, carbonaceous schist, calc-silicate rock, amphibolite and quartzite, deformed and metamorphosed during emplacement of the granitic to dioritic Nimbuwah Complex at 1867–1857Ma. The Cahill Formation hosts several world-class uranium deposits including Ranger, Jabiluka and Nabarlek. U–Pb SHRIMP and LA-SF-ICPMS detrital zircon spectra for four samples of the Cahill Formation and six samples of the Nourlangie Schist show a similar broad spectrum of ages mainly in the range 3300–1900Ma. A ubiquitous dominant peak at 2530–2470Ma matches the age of underlying Neoarchean basement, but is distinct in its dominantly mantle-like Hf and O zircon isotopic character, which shows closer similarity with possible source rocks from the Gawler Craton or alternatively from the Dharwar Craton. Common smaller age peaks occur at 2180Ma, 2080Ma and 2020Ma. The first two have no known magmatic age correlatives in the North Australian Craton. Zircons of the 2020Ma peak have distinctively unradiogenic Hf and elevated δ18O, at variance with local rocks of this age but similar to detrital zircon of the same age from the Gawler Craton. In contrast to younger Proterozoic sedimentary successions within the Pine Creek Orogen, which contain ubiquitous ca. 1870Ma detritus, the detrital spectra for the Cahill Formation and Nourlangie Schist contain almost no ca. 1870Ma detritus. A maximum deposition age of ca. 1866Ma indicates deposition within error of intrusion of the Nimbuwah Complex. We propose that the Cahill Formation and Nourlangie Schist were deposited at a plate margin immediately prior to convergent tectonism. This resulted in their burial, deformation and amphibolite facies metamorphism during orogenesis associated with the Nimbuwah Event. These findings have implications for understanding the Paleoproterozoic evolution of the Pine Creek Orogen within the context of northern Australia.

SHRIMP U-Pb zircon geochronology and Nd-Sr isotopic study of the Mamianshan Group: implications for the Neoproterozoic tectonic development of southeast China

Northwestern Fujian contains abundant well-studied Precambrian basement, and was a composite terrane in Cathaysia during the Neoproterozoic; however, its magmatic activity, petrogenesis, and tectonic evolution remain controversial. This article focuses on the geochronology and geochemistry of the Neoproterozoic Group in order to resolve the above problems. We provide new SHRIMP U-Pb zircon dating for the Mamianshan Group: 851.9 ± 9.2 to 825.5 ± 9.8 Ma for the Longbeixi Formation, 796.5 ± 9.3 Ma for the Dongyan Formation, and 756.2 ± 7.2 Ma for the Daling Formation. These ages document the existence of Neoproterozoic magmatism in the northwestern Cathaysia Block. Dongyan Nd-Sr isotopic data show that mafic amphibolite schists, mafic greenschists, and quartzofeldspathic schists were derived from a more depleted mantle (initial εNd ˜ +5.5 and 87Sr/86Sr ratio 0.703409643), a mixture of depleted mantle and crustal components (initial εNd ˜ −1 and 87Sr/86Sr ratio 0.702045282–0.704147714), and late Palaeoproterozoic continental crustal materials (initial εNd < −1 and 87Sr/86Sr ratio 0.71083603), respectively. These new data, together with previous studies, suggest a bi-subduction-collision orogenic model for the Neoproterozoic evolution of the Yangtze and Cathaysia blocks. Our plate tectonic scenario involves earlier NW-dipping subduction during 1.0 Ga–860 Ma along the southeastern margin of the Yangtze Block and later NW-dipping subduction near the northwestern margin of the Cathaysia Block starting at ca. 850 Ma. The 796.5 ± 9.3 Ma age of the volcanic Dongyan Formation suggests that the final assembly of the Yangtze and Cathaysia blocks probably occurred after ca. 800 Ma. The 756.2 ± 7.2 Ma age of the Daling Formation indicates that post-orogenic extensional magmatism took place after 800 Ma along the northwestern margin of Cathaysia.

A Previously Unreported Locality Record for the Gila Monster (Heloderma suspectum)

Although the Gila Monster (Heloderma suspectum) is widely distributed throughout the Sonoran and portions of the Mojave Deserts of the southwestern United States and northwestern Mexico, details of its distribution in California are imperfectly known, due to the apparent rarity of the species in that state. In their review of Gila Monster records for California, Lovich and Beaman (2007) documented only 26 credible sightings during a period of 153 years. In May 2009 another sighting in California was documented by Ruppert (2010a, b) who photographed a specimen in the Providence Mountains, an area known for previous Gila Monster sightings (Lovich and Beaman 2007). In this paper we report the 28 credible record of a Gila Monster in California. In their review of factors affecting the distribution of Gila Monsters in California, Lovich and Beaman (2007) noted that all previous records except one were east of approximately 2116u longitude, corresponding to that portion of the eastern Mojave Desert that receives more than 24 percent of annual precipitation during the warm season (1 July–14 October). They also noted that the majority of records with dates represented observations in April and May. Habitat varied but most observations occurred in mountainous terrain with rocky, incised topography ranging from near sea level to over 1,200 m. Many sightings were in riparian or xeroriparian areas. Based on the ecological attributes considered important in the distribution of the species, Brown and Carmony (1991) and Lovich and Beaman (2007) predicted that future sightings might be reported from the Granite, Whipple, Turtle, and Chemehuevi Mountains of California. To date, that prediction has not been fulfilled to the best of our knowledge. While conducting geological mapping for his Ph.D. dissertation (Haxel 1977), the junior author observed a Gila Monster at close range on the south side of Black Mountain in the southern Chocolate Mountains of southeasternmost California (Fig. 1), at an elevation of 300 6 60 m. This area is underlain mainly by quartzofeldspathic schist (Orocopia Schist), sandstone and argillite (Winterhaven Formation), rhyolite porphyry, conglomerate, basalt, sand and gravel, and colluvium. Much of the landscape is dominated by resistant outcrops and boulders of the basalt that caps Black Mountain. Vegetation, typical of the lower Sonoran Desert, is dominated by sparse creosote bush and cacti, with small ironwood and palo verde trees along drainage ways and washes. The date was 30 April, 1974, during the time of year when most Gila Monsters have been observed in California. The sighting location lies within the rectangle bounded by UTM gridlines E 5 0701000 and 0703000, N 5 3656000 and 3658000 (where E and N denote

Cretaceous age, composition, and microstructure of pseudotachylyte in the Otago Schist, New Zealand

At Tucker Hill, in Central Otago, New Zealand, a series of pseudotachylyte veins are hosted in quartzofeldspathic schist. Chilled margins, microlites, flow banding, and the crystallisation of mineral phases absent from the host rock provide unequivocal evidence for melting during pseudotachylyte formation. Whole rock analyses of pseudotachylyte reveal c. 3× enrichment of K2O, Ba, and Rb, and similar depletion of Na2O, CaO, Sr, and Eu, as compared to host schist. Formation age of pseudotachylyte is 95.9±1.8 Ma as measured by total fusion 40Ar/39Ar analyses. Stepwise heating of pseudotachylyte matrix yields an excellently defined 40Ar/39Ar plateau age of 96.0±0.3 Ma. These well-defined ages are attributed to the presence of potassium feldspar, low abundance of inherited lithic material from the host rock, and few fluid inclusions containing extraneous Ar. We propose that formation of these pseudotachylyte veins was related to Cretaceous extensional uplift and exhumation of the Otago Schist.

The onset of Pacific margin accretion in NE China: Evidence from the Heilongjiang high-pressure metamorphic belt

The Heilongjiang Complex is a sequence of high-pressure metamorphic rocks, located along the suture zone that separates the Jiamusi–Khanka (–Bureya) and Songliao–Zhangguangcai blocks in NE China (and extending northward into Far East Russia). The complex consists of mafic–ultramafic rocks, various quartzo–feldspathic schists and radiolarian-bearing quartzite (formerly chert). The rocks were metamorphosed up to epidote–blueschist facies, with P– T conditions of approximately T = 320–450 °C and P = 0.9–1.1 GPa. The lithological association and major and trace element compositions indicate that the blueschists were metabasalts of OIB and E-MORB affinity, most likely generated in a rift setting at the western margin of the Jiamusi Block that later underwent subduction. Magmatic zircons extracted from two samples of epidote–blueschist facies metabasalts from Mudanjiang have SHRIMP U–Pb 206Pb/ 238U ages of 213 ± 2 Ma and 224 ±7 Ma, whereas similar rocks ∼ 200 km farther north at Yilan have ages of 258 ± 2 Ma and 259 ± 4 Ma. These data define the protolith ages of the metabasalts as Late Triassic and Late Permian, respectively. These ages limit the timing of high-pressure metamorphism in the Heilongjiang Complex to post-Late Triassic, consistent with argon data reported from previous studies. Inherited zircon components in all four epidote–blueschist facies samples show distinct populations at 290–330 Ma, 420–530 Ma, 670–910 Ma and > 1065 Ma. Such ages are also a feature of the Central Asia Orogenic Belt (CAOB) to the west, supporting the view that the Jiamusi Block was most likely the rifted easternmost segment of the CAOB and not an exotic block derived from Gondwana. Final closure between the Jiamusi–Khanka–Bureya and Songliao blocks took place in the latest Triassic to Early Jurassic, with the two blocks accreted as a result of Pacific Ocean subduction. This suggests that the Heilongjiang Complex records the time when northward movement of the combined Mongolia–North China Block toward Siberia was waning and becoming surpassed by the onset of Pacific accretion from the east, which has dominated the tectonics of NE China and Far East Russia since the Early Jurassic.

Crystal fractionation in the friction melts of seismic faults (Alpine Fault, New Zealand)

Compositional variations are documented in friction melts along the Hari Hari section of the Alpine Fault, New Zealand, with multiple stages of melt injection into quartzo-feldspathic schists. Intermediate to felsic melts were heterogeneous in composition, but all fractions show a common trend, with a tendency for the younger melt layers and glasses to be more alkali − (Na + K) and Si-enriched, while being depleted in mafic (Fe + Mg + Mn) components. These changes are attributed primarily to crystal fractionation of the melt during transport. Farther traveled molten layers were on the whole less viscous, mostly due to a higher melt-to-clast ratio; however, compositional change, together with a decrease in volatile content, produced a progressively more viscous liquid melt with time. The glass phase is interpreted as a remnant of this high viscosity felsic residual melt that was preserved during final quenching. Following initial failure, the formation of largely phyllosilicate-derived, volatile-rich, lower viscosity melt corresponds with a phase of fault weakening. Subsequent rapid crystal fractionation during melt transport, the loss of volatiles and freezing of residual melt contributed to the strengthening of the fault during seismic slip.

Pseudotachylyte-generating faults in Central Otago, New Zealand

In Central Otago, New Zealand, a series of subparallel, NNE–SSW trending, gently dipping (10–30° E) brittle faults are hosted in quartzofeldspathic schist. These faults contain numerous ( n>100) pseudotachylyte veins. Pseudotachylyte fault veins are traceable along strike for distances of up to 200 m, and are between <1 mm and 5 cm thick. Fault separations inferred from offset quartz veins, dilational jogs and a piercing point analysis suggest that pseudotachylyte was generated by slip increments of <1 cm to 20 cm, comparable to those believed to occur during small to moderate-sized earthquakes [Sibson, R.H., 1989. Earthquake faulting as a structural process. Journal of Structural Geology, 11 (1–2) 1–14.]. Dilational jogs, fault drag and quartz vein separations imply a top-to-the-north (normal) sense of shear. Evidence for melting is provided by chilled margins, microlites and the formation of mineral phases not present in the host rock. Melting temperatures of 900–1100 °C are inferred from petrological observations. Heat-work calculations, utilising displacements on pseudotachylyte-bearing faults, and inferred melting temperatures suggest that the faulting occurred at depths of 6–12 km, with average frictional shear resistance exceeding 100 MPa on pseudotachylyte-generating faults. These inferred depths and relatively high levels of frictional shear resistance on pseudotachylyte-generating faults are similar to those inferred by some previous studies of pseudotachylyte (e.g. [Sibson, R.H., 1975. Generation of pseudotachylyte by ancient seismic faulting. The Geophysical Journal of the Royal Astronomical Society, 43 (3) 775–794; Killick, A.M. and Roering, C., 1998. An estimate of the physical conditions of pseudotachylite formation in the West Rand Goldfield, Witwatersrand Basin, South Africa. Tectonophysics, 284 (3–4) 247–259.]).

Depositional setting and paleogeographic implications of earth's oldest supracrustal rocks, the &gt;3.7Ga Isua Greenstone belt, West Greenland

Abstract New structural and stratigraphic mapping in the Isua greenstone belt, West Greenland has revealed that the exposed lithologic succession is quite different to that depicted in earlier studies. The key to a better understanding of the stratigraphy has been the recognition of intense strain and metasomatic effects combined with ductile fault structures that segment the belt into a number of tectonic slices. In some of these slices, deformation and metasomatism are somewhat lower than in surrounding slices (though still significant), which permit the recognition of primary depositional features that may be used to compare with lithologies from elsewhere in the belt. The belt is dominated by amphibolite that in a number of places show well-defined pillows. Pillow breccias and basaltic debris flows also occur within this package. Strongly recrystallized ultramafic bodies that occur in the belt are interpreted as intrusions or komatiitic flows. The most common sedimentary rock type is chert/banded iron-formation. These lithologies have been strongly affected by brittle and ductile deformation in combination with coarse recrystallization. Siliciclastic detrital rocks such as conglomerate and sandstone are much less common in the belt, and where present, have intrabasinally derived sources. Highly deformed quartzo-feldspathic schist crops out in a number of places in the belt and most likely has multiple origins. Previously these schists have been considered to be felsic volcaniclastic rocks, though recent mapping, and geochemical, and isotopic studies support the hypothesis that at least some of these schists are highy deformed and carbonated tonalitic gneiss sheets or replaced pillow-lava successions. Carbonate rocks in the belt are now considered to be mostly, or entirely, replacement in origin, where metasomatizing fluids have particularly utilized amphibolite–chert contacts; additionally, some carbonate may represent the products of early sea-floor alteration. We envision a temporarily emergent, below wave base, sea-floor origin for the succession based on the assemblage of lithologies.

Soil chronosequences in subalpine superhumid Cropp Basin, western Southern Alps, New Zealand

Soil chronosequences are described from Cropp River, western Southern Alps, New Zealand. In this area of high rainfall and bedrock of quartzofeldspathic schist, soils are developed in alluvium on gentle slopes and colluvium on steeper slopes, and form under indigenous grassland or scrubland, respectively. Soil ages ranging from 70 to c. 10 000 yr were determined from dendrochronology, radiocarbon dating, and stratigraphic relationships. Soil morphological and chemical changes with time reveal a similar soil development sequence on both gentle and steep slopes. Soil development is rapid and progressively forms Fluvial or Orthic Raw Soils (<200 yr), Fluvial or Orthic Recent Soils, Acid Brown Soils and Orthic Podzols (500–1500 yr), and Perch‐gley Podzols (7000–10 000 yr). Strong leaching is indicated by rapid acidification of surface horizons in Recent Soils. Morphological and chemical evidence shows that older soils (>1000 yr) are podzolised and gleyed. Eluvial/illuvial coefficients calculated from total element analyses, using Zr as an internal standard, indicate large relative losses of all elements from A and E horizons, with the greatest losses in the older soils. The formation of low‐permeability silt loam E horizons, as a result of strong weathering of alluvium and colluvium, causes the development of perch‐gley conditions in soils on very steep slopes and in relatively young soils that do not have iron pans. As a result of the very high rainfall, soil development is more rapid than described from anywhere else in Westland.