Igneous Units Research Articles

Temporal evolution of the Permian large igneous province in Tarim Basin in northwestern China

The temporal relations of different types of rocks in the Tarim large igneous province (TLIP) are studied using stratigraphic correlation, lithologically spatial distribution and isotopic ages. The TLIP covers an area >250,000 km 2 in the western and central part of the Tarim Basin. The lithological units of the TLIP include basalt, diabase, layered intrusive rock, breccia pipe mica-olivine pyroxenite, olivine pyroxenite, gabbro, ultramafic dyke, quartz syenite, quartz syenite porphyry and bimodal dyke. This is the first report for the spatial distribution of the TLIP from the spatial section lines, which shows the stratigraphic correlation among basaltic lava, tuff, interlayered mudstone, siltstone and sandstone and the thicknesses of the basaltic lavas from different field sections and drill holes, and the basalts from the Kupukuziman and Kaipaizileike Formations were subdivided in the drill hole sections. This indicates that the basaltic lavas were widely distributed in the Tarim Basin. Combined with previous study of geochemical and Sr–Nd–Pb isotopic characterization from the basalts and their genetic link with mantle plume activity, and large diabase swarms developments, the Permian igneous units constitute a Tarim large igneous province. Zircon SHRIMP U–Pb dating firstly yields 284.3 ± 2.8 Ma for the quartz syenite porphyry. Recent SHRIMP and LA-ICP-MS zircon U–Pb and 40Ar/ 39Ar ages are used to discuss temporal relations of different rock units in the TLIP formed between 290 and 274 Ma, and these data are more reliable than the K–Ar ages of 220–310 Ma reported previously. The sequence of magmatism of the TLIP in the central and western parts of the Tarim Basin are basaltic lava in the Kupukuziman and Kaipaizileike Formations (285–290 Ma), layered mafic–ultramafic rock, mica-olivine pyroxenite breccia pipe, diabase and ultramafic dyke, quartz syenite, quartz syenite porphyry and bimodal dyke (274–284 Ma).

In situ U-Pb geochronology of baddeleyite-zircon pairs using laser-ablation ICPMS: the case-study of quartz gabbro from Varney Nunatak (central Victoria Land, Antarctica)

In situ U-Pb geochronology was carried out by laser-ablation ICPMS on baddeleyite and zircon from a quartz gabbro of the Varney Nunatak intrusion (central Victoria Land, Antarctica). Back-scattered electron images reveal that baddeleyite grains from the investigated rock are partially substituted by zircon. Baddeleyite replacement by zircon is attributed to a late magmatic reaction between early fractionating minerals (plagioclase + sub-ophitic clinopyroxene + ilmenite + baddeleyite) with a silica oversaturated melt enriched in volatiles and incompatible elements. The development of fine-grained aggregates of clinopyroxene + amphibole + ilmenite around sub-ophitic clinopyroxene provides further evidence for this reaction. Homogeneous domains of baddeleyite were analysed by laser-ablation ICPMS and yielded crystallisation ages for the quartz gabbros of 488 ±3 Ma. U-Pb analyses on zircon define a population at 486 ± 6 Ma, coeval with the U-Pb baddeleyite crystallization age, and a population at 525 ± 10 Ma, representing an inherited component. This finding indicates that the parent magma of the quartz gabbro was contaminated with upper crustal rocks. The gabbroic rocks of Varney Nunatak are associated with peraluminous granites and late basalt dykes. The similar whole-rock and clinopyroxene major and trace element concentrations in the quartz gabbros and basalts is suggestive for a genetic relationship. The emplacement age, whole-rock compositions and highly enriched Nd isotope signature (ɛ Nd (488 Ma) of −7.7 to −6.8) of the Varney Nunatak association are similar to those of other igneous units from central Victoria Land. The association developed during the post-collisional phase of the Ross orogeny.

Upper Jurassic radiolarians from the Naga Ophiolite, Nagaland, northeast India

Radiolarians, extracted from cherts collected from an ophiolitic mélange near Salumi, Nagaland, NE India, have well-preserved tests and can be assigned to the Upper Jurassic (Kimmeridgian–lower Tithonian). These are the first well-preserved and clearly imaged radiolarians reported from the Naga Ophiolite. They are significantly older than fossils previously reported from this mélange, and their ages are similar to those determined radiometrically from associated igneous units.

Origin and emplacement of Archean ophiolites of the central orogenic belt, North China craton

Understanding Archean crustal and mantle evolution hinges upon identification and characterization of oceanic lithosphere. We report and update here more than 10 years work on a complete, yet dismembered and metamorphosed Archean ophiolite sequence in the North China craton, in the Dongwanzi (东湾子)-Zunhua (遵化) structural belt and correlatives in the Wutaishan (五台山) area. Banded iron formation structurally overlies several tens of meters of variably deformed pillow lavas, mafic flows, and picritic amphibolites. These are in structural contact with a 2 km thick mixed gabbro and dike complex with gabbro screens, exposed discontinuously along strike for more than 20 km. The dikes consist of metamorphosed diabase, basalt, Hb-Cpx-gabbro, and pyroxenite. The dike/gabbro complex is underlain by several kilometers of mixed isotropic and foliated gabbro, which preserve compositional layering approximately 2 km below the dike complex, and then over several hundred meters merge into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone into cumulate ultramafic rocks including serpentinized dunite, pyroxenite and wehrlite, and finally into strongly deformed and serpentinized olivine and orthopyroxene-bearing ultramafic rocks interpreted as depleted mantle harzburgite tectonites. A U/Pb zircon age of 2.505 Ga from gabbro of the Dongwanzi ophiolite makes it one of the world’s oldest recognized, laterally-extensive complete ophiolite sequences, though older dismembered ophiolites are recognized in South Africa and Greenland, extending back to 3.8 Ga. This age is confirmed by a ca. 2.6 Ga Re-Os isochron from chromites from the belt, and a number of dated 2.5–2.4 Ga cross-cutting younger igneous units. The Dongwanzi ophiolite is one of the largest well-preserved greenstone belts in the central orogenic belt that divides the North China craton into eastern and western blocks. More than 1 000 other fragments of gabbro, pillow lava, sheeted dikes, harzburgite, and podiform-chromite bearing dunite occur as tectonic blocks (tens to hundreds of meters long) in a biotite-gneiss and BIF matrix, intruded by tonalite and granodiorite, in the Zunhua structural belt. Blocks in this metamorphosed Archean ophiolitic melange preserve deeper levels of oceanic mantle than the Dongwanzi ophiolite. The ophiolite-related melange marks a suture zone across the North China craton, traced for more than 1 600 km along the central orogenic belt. Many of the chromitite bodies are localized in dunite envelopes within harzburgite tectonite, and have characteristic nodular and orbicular chromite textures, known elsewhere only from ophiolites. The chromites have variable but high chrome numbers (Cr/(Cr+Al)=0.74−0.93) and elevated P, also characteristic of suprasubduction zone ophiolites. The high chrome numbers, coupled with TiO2<0.2 wt.% and V2O5<0.1 wt.% indicate high degrees of partial melting from a very depleted mantle source and primitive melt for the chromite. A Re-Os isochron from the chromites indicates an age of 2.6 Ga, showing that they are the same age as the Dongwanzi ophiolite. The range in initial Os isotopic compositions in the chromites in these ophiolitic blocks is small and well within the range seen in modern ophiolites. The ultramafic and ophiolitic blocks in the Zunhua melange are therefore interpreted as dismembered and strongly deformed parts of the Dongwanzi ophiolite. We suggest the name “Dongwanzi-Zunhua ophiolite belt” for these rocks. Geochemical and structural features of the Dongwanzi ophiolite suggest that it formed in a forearc environment and was incorporated in an accretionary prism soon after it formed. Neoarchean and Paleoproterozoic (2.50 and 1.90 Ga) high-pressure granulites form a belt more than 700 km long along the western side of the central orogenic belt. Several Neoarchean sedimentary basins consisting of conglomerate, greywacke, and shale are located along the eastern side of the central orogenic belt, and are interpreted as remnants of a foreland basin. The three belts record the Neoarchean subduction and collision between an arc terrane and eastern blocks of the North China craton in the Neoarchean, and further deformation and metamorphism in the Paleoproterozoic related to collisions on the northern margin of the already amalgamated North China craton.

Geochemistry and petrology of late Eocene Cascade Head and Yachats Basalt and alkalic intrusions of the central Oregon Coast Range, U.S.A.

The Oregon Coast Range underwent extension and basaltic magmatism accompanying tectonic rotation during the Tertiary. Basaltic magmatism was expressed initially in submarine eruptions, but mostly in younger subaerial flows. A tholeiitic basalt series, represented by Yachats units, evolved from a depleted, low K, Rb source through a process of fractional crystallization and magma mixing. An alkalic basalt series, represented by Cascade Head units, evolved primarily by fractional crystallization from parental magmas created by lower degrees of partial melting at greater depth than the Yachats series. Camptonite, found in sills and dikes, was formed by even smaller degrees of melting of sources similar to those of Cascade Head. Small nepheline syenite intrusions, emplaced as sills into middle Eocene Tyee Formation, appear to have formed through extensive fractional crystallization of parental basanite, which is sparsely represented in the Yachats and Cascade Head basaltic lava units. Individual syenite intrusions are not directly related to each other, as each intrusion represents a relatively small batch of magma separated at different evolutionary stages from underlying magma chambers. P–T segregation conditions were estimated for primitive Coast Range basalts. These include depths of ~ 120–160 km and temperatures of 1600–1700 °C for the alkalic series represented primarily by Cascade Head units and depths of ~ 60–120 km with temperatures of ~ 1500–1600 °C for tholeiitic units. A few analyses from other Coast Range igneous units (Tillamook and Goble volcanics) suggest T–P segregations at shallower depths (~ 55–33 km) and lower temperatures (~ 1400 °C). Paired Nd and Sr isotopic analyses of four Coast Range samples fall in the enriched MORB field. The depths and high temperatures indicated for the Central Oregon Coast ancestral basalts require magma generation in the garnet peridotite stability field, with the alkalic series probably originating below the subducting Juan de Fuca plate. This would suggest an origin for the magmas in a slab window or mantle plume environment. We further suggest that major extension documented for the Coast Range may have triggered upwelling of mantle asthenosphere giving birth to a mantle plume.

A microstructural imprint of melt impregnation in slow spreading lithosphere: Olivine‐rich troctolites from the Atlantis Massif, Mid‐Atlantic Ridge, 30°N, IODP Hole U1309D

The 1415 m deep IODP Hole U1309D (Mid‐Atlantic Ridge, 30°N) is the second deepest hole drilled into slow spreading oceanic lithosphere. The recovered section comprises essentially gabbroic rocks, with a large range of compositions. The most primitive end‐members of the gabbroic sequence, herein referred to as olivine‐rich troctolites (ol &gt; ∼70%), have textures and geochemical compositions intermediate between that of mantle peridotites and primitive cumulates, indicative of melt impregnation processes. We carried out a detailed microstructural study to further characterize the petrogenetic processes leading to their formation, as well as discuss their mode of emplacement and relationship with neighboring mantle lithosphere. In olivine‐rich troctolites, olivines range from coarse‐grained subhedral crystals, commonly containing well‐developed subgrains, to medium‐grained rounded crystals with fewer or no substructures. They are embedded in large, undeformed pyroxene and plagioclase poikiloblasts. Olivine substructures reveal dislocation creep that is consistent with activation of the main high‐temperature slip systems, dominantly (010)[100]. Olivine crystallographic preferred orientation is very weak but generally shows a relatively stronger, uncommon [001] concentration. These unusual olivine fabrics are interpreted as resulting from melt impregnation of a previously deformed olivine matrix: the solid olivine framework is disrupted by olivine corrosion along grain and subgrain boundaries, and the high‐temperature plastic fabric is modified in a liquid‐dominated regime. Based on mineral composition and fabrics and in comparison with what is observed in impregnated mantle rocks elsewhere, we posit that olivine represents relicts of mantle peridotites disaggregated by large melt influx, although the mantle origin of olivine is not unequivocally demonstrated yet. Whatever the initial lithology, impregnation by large volumes of melt has strongly modified the original composition and microstructure. If the mantle origin hypothesis is correct, the original olivine fabric could have been efficiently weakened by dunitization prior to disruption of the olivine framework by melt impregnation. Incorporation, at the base of the lithosphere, of small slivers of impregnated dunite into gabbroic sections, trapped between successive igneous units, may be a common mechanism of lower crustal accretion at slow spreading ridges. Extensive melt‐rock interaction processes are expected to contribute significantly to the final chemical composition of erupted lavas.

Oligocene-Miocene basin evolution in the northern Altiplano, Bolivia: Implications for evolution of the central Andean backthrust belt and high plateau

The upper Oligocene to lower Miocene Penas and Aranjuez formations are exposed in north-northwest–trending outcrop belts of the central Andean backthrust belt situated within the central Andean plateau along the boundary between the northern Altiplano and the Eastern Cordillera of Bolivia. Sedimentary lithofacies analyses indicate that these coarse-grained siliciclastic formations were deposited primarily in alluvial fan to braided fluvial environments. An upsection change from principally fine-grained sandstone to cobble conglomerate is consistent with increased proximity to the sediment source with time. Paleocurrent analyses reveal that flow was predominantly directed toward the west-southwest away from Cordillera Real, the elevated core of the Eastern Cordillera. Provenance data from conglomerate clast compositions and sandstone petrofacies suggest derivation from recycled quartz-rich metasedimentary and sedimentary rocks from the Paleozoic section in the Eastern Cordillera. The paleoflow orientations, sediment provenance, and increased proximity of the sediment source suggest that deposition of the Penas and Aranjuez formations was related to surface uplift of the Eastern Cordillera relative to the Altiplano. Growth strata observed in the Aranjuez Formation further indicate that shortening was synchronous with deposition, probably in a hinterland basin. New 40 Ar/ 39 Ar ages from a lowermost exposed igneous unit and interbedded ash-fall tuff beds in the Aranjuez and Penas formations show that synorogenic sedimentation and fold-thrust deformation in the frontal (west-southwestern) zone of the central Andean backthrust belt was concentrated during late Oligocene–early Miocene time. These age results are consistent with previous studies of east-derived sedimentation in the Altiplano and indicate regional uplift of the Eastern Cordillera at this time. Upsection trends in provenance data further suggest a progressively greater contribution from younger Paleozoic strata, possibly due to activation of new thrust faults during west-southwestward propagation of the backthrust belt toward the Altiplano. Such a progression of late Oligocene–early Miocene shortening along the Altiplano–Eastern Cordillera boundary likely reflects significant crustal thickening, potential isostatic uplift, and initial topographic expression of the eastern margin of the central Andean plateau.

Mapping Mars geochemically

Using multivariate cluster analysis, we identify six distinct geochemical provinces on Mars from the concentrations of K, Th, Fe, Si, Ca, Cl, and H 2 O determined by the Mars Odyssey gamma ray spectrometer. The results show that the Martian surface is dominated by basaltic rocks that vary in their abundance of incompatible (K, Th) and major elements (Fe, Si, and Ca). These chemically distinct geochemical provinces are in large, contiguous regions comprising a mixture of geologic units. The K/Th ratios are uniform among the geochemical provinces. To prevent measurable fractionation of K from Th, aqueous events must have been brief and/or the total throughput of water small. The muted weathering effects led to deposition of younger sedimentary deposits with the same compositions as older igneous units, explaining why a geochemical province may contain mapped units with the same composition but a range of ages.

REE and HFSE mobility due to protracted flow of basinal brines in the mesoproterozoic Belt-Purcell Supergroup, Laurentia

Argillites from the Belt-Purcell Supergroup record three types of REE patterns. Type 1 (T1), near-primary compositional pattern is characterized by: (1) a PA-UCC-like REE pattern, and Th/Sc ratios reflecting a dominantly Paleoproterozoic catchment; (2) depletion of Ca, Sr and Na due to weathering in the provenance of the sediments; (3) variable depletion of Nb–Ta and Zr–Hf from winnowing of titanite and zircon, respectively, with Nb/Ta and Zr/Hf ratios similar to PA-UCC; and (4) K-enrichment, with variable Th/U and Ce/Ce* ratios from diagenesis. Sandstones with T1 patterns are compositionally equivalent to argillites diluted with quartz, and show the same four T1 compositional features, albeit at lower absolute abundances. Relative to T1 argillites, Type 2 (T2) and Type 3 (T3) patterns both feature enriched and fractionated HREE, but T2 exhibits LREE depletion. Relative to T1 these constitute T2 low − ΣLREE/ΣHREE (15 ± 3) and T3 high − ΣLREE/ΣHREE (24 ± 3) patterns showing progressive enrichment of HREE, erratic ratios of Zr/Hf, Ti/Sm, Y/Ho, Y/Yb and Al/Yb. Collectively, T2 and T3 patterns record diagenetic transfer of REE as well as REE/HFSE and HFSE/HFSE fractionations. Such fractionations are more pronounced in sandstones due to greater hydraulic conductivity for diagenetic fluids. Hence REE and HFSE patterns in the argillites and sandstones are interpreted as diagenetic processes superimposed on a PA-UCC provenance signature, specifically to pervasive and episodic migration of oxidizing-alkaline diagenetic basinal brines in the Belt-Purcell Supergroup. During these diagenetic processes Al, Th and Ga behaved isochemically. Dominantly positive Ce anomalies coupled with mobility of U (as recorded by variable Th/U ratios) is consistent with oxidizing conditions, which may have been internally generated within the basin by reactions between clays and carbonate. Alkalinity of the fluids could have been promoted by some combination of: (1) enrichment of K as indicated by mass balance calculations and the presence of illite; (2) hydrothermal and/or diagenetic alteration of mafic igneous units lower in the stratigraphic sequence; and (3) dissolution of evaporitic units, evident in relict halite casts, in the Belt-Purcell sequence. Supporting this interpretation, previous studies revealed two populations of monazite, differentiated in terms of texture, composition and age. Detrital monazites are >1400 Ma, whereas diagenetic monazites are euhedral and chemically distinctive, spanning ∼1400–∼300 Ma. The diagenetic monazites may record episodic brine activity for ∼1000 Ma that induced T2 and T3 compositional features, and redistributed HREE and Zr–Hf at a basin scale.

Using sediment geochemistry and detrital zircon geochronology to categorize eroded igneous units: An example from the Mesoarchean Birch-Uchi Greenstone Belt, Superior Province

Sediments deposited in volcanically active areas preserve a record of the geochemistry of eroded igneous units. In some instances this record can be more complete than the remaining volcanic rocks. Whole rock geochemistry and detrital zircon geochronology were performed on sedimentary rocks of the Balmer assemblage in the Birch-Uchi Greenstone Belt, western Superior Province, to investigate the types and ages of igneous rocks that were present in the source area at approximately 2.9 Ga. Spatially juxtaposed Mesoarchean volcanic rocks were also analyzed for comparative purposes. The detrital zircon geochronology revealed that the sandstones studied are significantly younger than the adjacent volcanic rocks, inferring that their stratigraphic position is the result of tectonic interleaving and they do not belong in the Balmer assemblage. As well, the majority of the detrital zircons were older than the adjacent volcanic rocks severely limiting the potential role of these volcanic units as a source terrain. Extreme geochemical heterogeneity within the sandstone sample set implies the depositional environment was supplied by multiple sediment fed systems, which probably delivered detritus to a submarine ramp environment. A subset of the sandstone samples are HFSE enriched and had, in part, an alkalic igneous source. This infers that the ocean plateau tholeiitic basalts, which are abundant in northwest Superior Province, may have been associated with alkalic eruptive units, similar to modern intra-plate igneous activity. Either erosive or tectonic processes have removed these alkalic igneous units from the preserved rock record, but their detritus is present and identifiable in the sedimentary succession.

Bulk chemical composition of lherzolitic shergottite Grove Mountains 99027—Constraints on the mantle of Mars

Abstract— We report the concentration of 50 elements, including rare earth elements (REEs) and platinum group elements (PGEs) in bulk samples of the Grove Mountains (GRV) 99027 lherzolitic shergottite. The abundances of REEs are distinctly lower than those of Allan Hills (ALH) A77005 and other lherzolitic shergottites, indicating that GRV 99027 is not paired with them. It may, nevertheless, sample the same igneous unit as the others (Lin et al. 2005b; Wang and Chen 2006). The CI‐normalized elemental pattern of GRV 99027 reveals low (0.004–0.008 × CI) and unfractionated PGEs (except for Pd of 0.018 × CI) without depletion of W. or Ga relative to lithophile element trends. Fractionation between siderophile and lithophile elements become less pronounced with increase of volatility, except for high abundances of Ni and Co. These characteristics are probably representative of the mantle of Mars, which is consistent with previous work that the Martian mantle formed in a deep magma ocean followed by a later accretion of chondritic materials.

The basaltic volcanic rocks in the Tuyon Basin, NW China: Petrogenesis and tectonic implications

The Tuyon basin is located near the crossing of the Tien Shan orogenic belt, the Western Kunlun fold system and the TarimAlashan blocks. The crossing of two lithospheric scale fault, the Talas-Ferghana dextral strike-slip fault and the NE extended South Tien Shan strike-slip fault, is just at the southern margin of the Tuyon basin. Therefore, many geoscientists all over the world have been attracted to petrogenesis of the Cenozoic basaltic volcanic rocks in the Tuyon basin and their tectonic implications. According to previous researches, the basalts emplaced within the Tuyon basin are of two units, the lower basalts and the upper basalts, aged 120~110 Ma and 70.4~36.6 Ma respectively. All these basalts are classified as alkali series, enriched in potassium, and considered as a result of a small plume probably rooted at a shallow level in the asthenosphere. Our new works on the igneous rocks in the Tuyon basin suggest that the so-called lower basalts are preserved as a series of cone dikes, which are emplaced in Jurassic-Lower Cretaceous sedimentary rocks; the upper basalts are formed as a series of volcanic eruptions, including calderas, ring dikes, necks and volcanic cones. For example, there are three volcanic cones in the line between the Kuvt cone and Bilebluk cone. We have discovered basaltic intrusive units in Jurassic sandstone and conglomerate strata. The basaltic extrusive units have the same occurrence with the early Cretaceous sediments, but uncomfortably overlain by the Paleogene sedimentary rocks. Therefore, the Tuyon basin is a compound volcanic basin, including series of volcanic eruptions with lavas and subvolcanic rocks, rather than it has two-layer structure consisted of two igneous units. Our investigations indicate that the basaltic rocks are mainly of dark gray, massive structure and porphyritic texture. The basaltic extrusive rocks are developed as columnar joint structure, vesicular structure and amygdaloidal structure. Most of volcanic rocks within Tuyon basin contain phenocrysts such as olivine, clinopyroxene and plagioclase, and occasionally phlogopite, amphibole and apatite. Olivine belongs to chrysolite circled by fine-grained aggregate of magnetite and talc. Most of olivine phenocrysts have been altered into iddingsite and talc. The clinopyroxene mainly belongs to augite, which characterize hourglass structure and zoned texture. The plagioclase phenocrysts have more content of anorthite. They are mainly andesine and occasional labradorite, but rare in zoned texture. The matrix minerals are clinopyroxene, plagioclase, magnetite and volcanic glass, and mainly formed tholeiitic texture, occasionally intergranular texture and vitrophyric. Therefore, the Cenozoic volcanic rocks in the Tuyon basin show characteristics of alkali series, and suggest that the region may was at extensional condition. In the Arhesaiordy volcanic-sedimentary sequences there are five basaltic sills intruded into the Jurassic sedimentary deposits. Many megacrysts have been found in the lowest basaltic layer, such as augite, phlogopite, amphibole and anorthoclasite. From the same layer we have collected lots of the deep-seated xenoliths including peridotite, granulite, pyroxenite, gneiss and so on. In the other four layers, the number of megacrysts and deep-seated xenoliths has been decreased quickly. There are also lots of deep-seated xenoliths in the Tasgeler caldera. Therefore, basaltic magma has the characteristics of rapid ascent. Consequently, the basaltic volcanic rocks in the Tuyon basin are formed direct from primary magma, which is rare affected by fractional crystallization.

UPPER TRIASSIC TO CRETACEOUS RADIOLARIA FROM NICARAGUA AND NORTHERN COSTA RICA - THE MESQUITO COMPOSITE OCEANIC TERRANE

We propose a new terrane subdivision of Nicaragua and Northern Costa Rica, based on Upper Triassic to Upper Cretaceous radiolarian biochronology of ribbon radiolarites, the newly studied Siuna Serpentinite Melange, and published 40Ar/39Ar dating and geochemistry of mafic and ultramafic igneous rock units of the area. The new Mesquito Composite Oceanic Terrane (MCOT) comprises the southern half of the Chortis Block, that was assumed to be a continental fragment of N-America. The MCOT is defined by 4 corner localities characterized by ultramafic and mafic oceanic rocks and radiolarites of Late Triassic, Jurassic and Early Cretaceous age: 1. The Siuna Serpentinite Melange (NE-Nicaragua), 2. The El Castillo Melange (Nicaragua/Costa Rica border), 3.The Santa Elena Ultramafics (N-Costa Rica) and, 4. DSDP Legs 67/84. 1. The Siuna Serpentinite Melange contains, high pressure metamorphic mafics and Middle Jurassic (Bajocian-Bathonian) radiolarites in original, sedimentary contact with arc-metandesites. The Siuna Melange also contains Upper Jurassic black detrital chert formed in a marginal (fore-arc?) basin shortly before subduction. A phengite 40Ar/39Ar -cooling age dates the exhumation of the high pressure rocks as 139 Ma (earliest Cretaceous). 2. The El Castillo Melange comprises a radiolarite block tectonically embedded in serpentinite that yielded a diverse Rhaetian (latest Triassic) radiolarian assemblage, the oldest fossils recovered so far from S-Central America. 3. The Santa Elena Ultramafics of N-Costa Rica together with the serpentinite outcrops near El Castillo (2) in Southern Nicaragua, are the southernmost outcrops of the MCOT. The Santa Elena Unit (3) itself is still undated, but it is thrust onto the middle Cretaceous Santa Rosa Accretionary Complex (SRAC), that contains Lower to Upper Jurassic, highly deformed radiolarite blocks, probably reworked from the MCOT, which was the upper plate with respect to the SRAC. 4. Serpentinites, metagabbros and basalts have long been known from DSDP Leg 67/84 (3), drilled off Guatemala in the Nicaragua-Guatemala forearc basement. They have been restudied and reveal 40Ar/39Ar dated Upper Triassic to middle Cretaceous enriched Ocean Island Basalts and Jurassic to Lower Cretaceous depleted Island arc rocks of probable Pacific origin. The area between localities 1-4 is largely covered by Tertiary to Recent arcs, but we suspect that its basement is made of oceanic/accreted terranes. Earthquake seismic studies indicate an ill-defined, shallow Moho in this area. The MCOT covers most of Nicaragua and could extend to Guatemala to the W and form the Lower (southern) Nicaragua Rise to the NE. Some basement complexes of Jamaica, Hispaniola and Puerto Rico may also belong to the MCOT. The Nicoya Complex s. str. has been regarded as an example of Caribbean crust and the Caribbean Large Igneous Province (CLIP). However, 40Ar/39Ar - dates on basalts and intrusives indicate ages as old as Early Cretaceous. Highly deformed Jurassic and Lower Cretaceous radiolarites occur as blocks within younger intrusives and basalts. Our interpretation is that radiolarites became first accreted to the MCOT, then became reworked into the Nicoya Plateau in Late Cretaceous times. This implies that the Nicoya Plateau formed along the Pacific edge of the MCOT, independent form the CLIP and most probably unrelated with he Galapagos hotspot. No Jurassic radiolarite, no older sediment age than Coniacian-Santonian, and no older 40Ar/39Ar age than 95 Ma is known from S-Central America between SE of Nicoya and Colombia. For us this area represents the trailing edge of the CLIP s. str.

Petrological and geochemical characteristics of metamorphic and igneous units from the allochthonous Madre de Dios Terrane, Southern Chile

The Denaro Complex, part of the Madre de Dios Terrane is composed of metamorphosed pillow basalts, metahyaloclastites, banded metalliferous and radiolarian metacherts, metapelites and redeposited calcareous metasandstones. The basaltic rocks show primary textures, minerals and structures. They are foliated especially in the vicinities of thrust faults, interpreted to have developed during the accretion of the terrane to the Gondwana margin. Composition of relic primary augite and chromite crystals plots into the MORB field of tectonic discriminant diagrams, as do the analyses of whole rock geochemistry, which indicates that these rocks are akin to volcanic rocks erupted along a constructive plate margin (N- and E-type MORBs), probably in a spreading axis-centered oceanic plateau or ridge. The metamorphic assemblages of pumpellyite–actinolite facies bear witness of metamorphism in a frontal accretionary wedge at elevated P and low T conditions, probably related to the Late Triassic–Early Jurassic Chonide event, which has been recognized elsewhere in the Patagonian Andes.

Evolution of the Namaqua-Natal Belt, southern Africa – A geochronological and isotope geochemical review

Juvenile crust formation within the Namaqua-Natal Belt occurred during two principal periods at ∼1.4 and ∼2.2 Ga with little evidence for significant contributions from older crustal sources. Palaeoproterozoic lavas and associated calc-alkaline granitoids are preserved in the Richtersveld sub-province and, to a much lesser extent, in the Bushmanland sub-province. Development of crust within the Aggeneys and Okiep terranes of the Bushmanland sub-province during the Mesoproterozoic involved significant reworking of pre-existing Palaeoproterozoic lithosphere, whereas the Garies terrane (Bushmanland sub-province) and the Gordonia and Natal sub-provinces show little or no reworking of older protoliths. Deformation of Archaean components is evident near the south-western margin of the Kaapvaal Craton but there is no evidence for a significant Eburnean (∼1.8 Ga) orogeny in the Kheis sub-province. Rather, the ‘Kheisian’ fabric is now dated as younger than ∼1.3 Ga, and is thus an early phase of the Mesoproterozoic evolution of the Namaqua-Natal Belt. Graben formation with associated extrusion of lavas and deposition of sediments of the Koras Group occurred relatively early in the history of the Belt, predating most of the intrusive granitoid activity evident in other sub-provinces. Early, juvenile, dominantly mafic to intermediate Mesoproterozoic igneous units formed at ∼1.2–1.3 Ga in the Gordonia and Natal sub-provinces. Two major periods of granitoid intrusion occurred at ∼1.15 and ∼1.03–1.08 Ga and were both of regional extent. The Little Namaqualand Suite intruded at ∼1.15 Ga in the Bushmanland sub-province, as did various individual plutons in the Gordonia and Natal sub-provinces. Spektakel, Keimoes and Oribi Gorge Suite granitoids, often megacrystic in character, were emplaced at ∼1.03–∼1.08 Ga. These latter granitoid suites each span several structural terranes, indicating that accretion of these domains was essentially complete by ∼1.03 Ga. Igneous activity as part of the Namaqua-Natal orogenesis was concluded by ∼1.0 Ga throughout the belt. Subsequent ∼0.85–0.75 Ga magmatism, evident only in the west, reflects the start of a new cycle which ultimately produced the Damara and Gariep belts. The dominant, penetrative regional fabric was produced prior to peak metamorphism at ∼1.02–1.04 Ga. Limited geochronological evidence also records an earlier phase of high-grade metamorphism at ∼1.16 Ga in both the western and eastern sectors of the Belt. Transcurrent shearing is dated at ∼1.06–1.03 Ga in the Natal sub-province. Equivalent shearing, although not directly dated, also occurred in the west as a result of indentor tectonics produced by collision with the Kaapvaal Craton. Subsequent exhumation and cooling of the rocks of the Namaqua-Natal Belt resulted in temperatures as low as ∼350 °C by ∼0.95 Ga. Except in the Gariep Belt and its foreland in the west, there is little evidence for subsequent, Pan-African overprinting or activity within the Namaqua-Natal Belt. The age of the supracrustal gneisses remains contentious. Early, ∼2 Ga metavolcanics and metasediments in the Richtersveld sub-province are presumably also present in parts of the Bushmanland sub-province because granitoids of that age occur in both sub-provinces. The Koeris amphibolites from the Bushmanland Group have provided a ∼1.65 Ga Sm–Nd date, whereas supracrustal gneisses in the Garies terrane (Bushmanland sub-province), Gordonia sub-province and Natal sub-province have provided ∼1.2 to ∼1.3 Ga dates. It therefore seems likely that supracrustal lithologies were deposited during at least three intervals as the Namaqua-Natal Belt developed.

A newly discovered orogenic event in Morocco: Neoproterozic ages for supposed Eburnean basement of the Bou Azzer inlier, Anti-Atlas Mountains

The Bou Azzer inlier within the Anti-Atlas orogenic belt of Morocco exposes a dismembered ophiolite, long considered to mark a Late Neoproterozoic suture between the West African Craton (WAC) in the south, and Neoproterozoic arcs to the north. Southern parts of the Bou Azzer inlier include meta-igneous units, paragneiss, schist and muscovite-bearing granite that have until now been regarded as the c. 2 Ga Eburnean WAC basement onto which the ophiolite was obducted. An example of supposed Paleoproterozoic basement occurs at Tazigzaout. The main units within this complex are metagabbro, augen granite gneiss, finely banded granite gneiss and variably sheared syn-tectonic leucogranite, all of which are intercalated within a complex sub-vertical, upper greenschist facies, ductile dextral shear zone. The meta-igneous rocks are crosscut by a sheeted muscovite-biotite granite pluton. Three concordant U–Pb analyses of zircon from an augen granite gneiss provide a date of 753 +1/−2 Ma. Zircon from a nearby metagabbro provide an indistinguishable date of 752.2 ± 2.4 Ma. Both dates are considered best estimates of the crystallization ages of their igneous protoliths. Analyses of zircon from two crosscutting leucogranite bodies provide younger dates of 705 +2/−3 Ma and 701 +2/−1 Ma. The initial ɛ Nd values of all lithological units in the complex range from +4.9 to +6.0, with depleted mantle model ( T DM) ages of 890–790 Ma. The high, positive ɛ Nd values for all of these igneous units is indicative of an isotopically juvenile source, such as depleted-mantle derived material, and precludes substantial interaction with old, evolved crustal sources. These data demonstrate that the “basement” rocks of the Tazigzaout complex are Neoproterozoic intrusions, not c. 2 Ga Eburnean units, as found in more southerly Anti-Atlas inliers. The structural relationships and U–Pb ages are interpreted as recording dextrally transpressive deformation of c.755 Ma juvenile crust after c. 750 and before 700 Ma. This Neoproterozoic deformation occurred as much as 100 Ma prior to the top-to-the-north thrusting of the meta-igneous units over units of the ophiolite and arc, and provides strong evidence for two discrete ‘Pan African’ events in Morocco.

Petrology, geochemistry, and geochronology of Proterozoic granitoid and related rocks of the northern Mummy Range, north-central Colorado

Detailed geologic mapping in the northern Mummy Range, north-central Colorado, reveals seven distinguishable igneous granitoid rock units intruded into a sequence of high-grade metamorphic host rocks. U-Pb dating of zircons from twelve samples indicates that these granitoids were emplaced during two separate periods of magmatism about 300 m.y. apart, one in the Paleoproterozoic and another in the Mesoproterozoic. Zircon dating confirms that all of this magmatic activity post-dated the peak of Paleoproterozoic metamorphism in this area. Based on field, geochemical and mineralogical analyses, four distinct Paleoproterozoic granitoid rock units are interpreted to represent three separate, but geochronologically indistinguishable, magmatic events at ∼1.7 Ga. The earliest igneous event is recorded by a body of leucocratic tonalite which yields a zircon age of 1702 ± 6 Ma. The second event is recorded by small bodies of granodiorite which are mineralogically and geochemically very similar to the approximately contemporaneous Boulder Creek Granodiorite exposed to the south. The third Paleoproterozoic igneous event produced foliated granitoid rocks that yield a composite zircon age of 1695 ± 20 Ma and which comprise two distinct lithologies: a melanocratic roof facies and a strongly foliated monzogranite. This foliated Paleoproterozoic monzogranite, which is geochemically and mineralogically indistinguishable from the approximately contemporaneous Rawah batholith exposed to the northwest, is the most widespread rock unit exposed in the northern Mummy Range, accounting for over 65% of the area of the Comanche Peak quadrangle north of the Skin Gulch shear zone. The earliest Mesoproterozoic igneous event in the northern Mummy Range is represented by small bodies of coarse-grained quartz diorite. The second Mesoproterozoic event is represented by the strongly porphyritic granodiorite of the Hagues Peak pluton. The third and final Mesoproterozoic igneous event in the area produced widespread bodies of equigranular “Silver-Plume-type” peraluminous monzogranite that yields a zircon age of 1393 ± 25 Ma. Most of our zircon ages for the Mummy Range granitoids are consistent with the most recent synthesis of models for the tectonic evolution of the Proterozoic Colorado province. Our dates for certain individual samples of the Paleoproterozoic granodiorite and foliated Paleoproterozoic monzogranite suggest that Paleoproterozoic syntectonic plutonism may have continued slightly longer in this area than previously recognized. The mineralogy and geochemistry of most of the Mummy Range granitoids are consistent with current tectonic syntheses of the region. The composition of the foliated Paleoproterozoic monzogranite in the Mummy Range suggests a source that included pre-existing continental crustal material, consistent with recent suggestions of

Short-lived magmatic activity in an anorogenic subvolcanic complex: 40Ar/ 39Ar and ion microprobe U–Pb zircon dating of the Erongo, Damaraland, Namibia

The Erongo subvolcanic center in Namibia is the largest composite, bimodal complex in the Mesozoic Etendeka igneous province of Namibia. This study of 40Ar/ 39Ar and high spatial resolution U–Pb zircon dating demonstrates that emplacement of the various igneous units at Erongo took place within a time span equivalent to or shorter than geochronologically resolvable age differences (ca. 2 Ma), and at the peak of regional flood-basalt activity in the Etendeka–Paraná province. The Erongo complex comprises a series of felsic volcanic and intrusive units that overlie or intrude basaltic lavas attributed to the Etendeka Group. The stratigraphically oldest rhyodacite (Erongorus unit) yielded U–Pb zircon ages of 131.8±1.1 Ma (1σ). The overlying Ombu rhyodacite was previously dated at 135.0±1.6 Ma and our new ages for a resurgent intrusion of the same magma type (Ombu granodiorite) in the vent area are 132.6±1.1 Ma ( 40Ar/ 39Ar) and 132.3±1.9 Ma (U–Pb zircon) respectively. The youngest felsic magmas at Erongo are the high-silica Ekuta rhyolite and compositionally equivalent Erongo granite. These units yielded U–Pb zircon ages of 131.9±2.9 and 130.3±1.4 Ma, respectively. Concordant 40Ar/ 39Ar biotite ages from two samples of the granite average 132.2±0.8 Ma. The final stage of magmatism at Erongo involved emplacement of basic alkaline plugs and dikes. Phlogopite and kaersutite from foidite plugs of this series yielded 40Ar/ 39Ar ages of 130.8±1.0 and 132.0±1.0 Ma. In a regional context, these age results indicate that silicic magmatism in the largest complexes of the Damaraland (Erongo, Brandberg, Paresis, Messum) began simultaneously with the peak of flood-basalt effusion at about 132 Ma throughout the Etendeka province and ceased by about 130 Ma. The silicic magmas are hybrid, with varying contributions of crustal and mantle-derived melts, and the age constraints suggest that crustal melting was caused by a short-lived thermal pulse related to the main flood-basalt event. Basic magmatism in the Damaraland complexes continued sporadically thereafter to about 123 Ma, but lacked the power to create further crustal melting.

Magmatic characteristics of the Paleocene Cerro Nevazón region and other Late Cretaceous to Early Tertiary calc-alkaline subvolcanic to plutonic units in the Neuquén Andes, Argentina

Voluminous Late Cretaceous–Early Tertiary calc-alkaline magmatism in the northwest part of the Neuquén province, Argentina, is characterized by a NS-trending belt of olivine basalt-augitic andesite volcanic and amphibole-rich diorite subvolcanic facies that were grouped in the Neuquén-Mendoza volcanic province. Of samples associated with skarn mineralization in the Cerro Nevazón area, two were dated by K/Ar in amphibole and one by Ar/Ar in plagioclase; together, they yield ages between 60.1±1.6 and 56.0±1.7 Ma. The igneous rocks contain normally zoned plagioclase (64–74%), amphibole (magnesiohastingsite and edenite; 15–20%), biotite (5–10%), quartz (4–18%), minor enstatite–ferrosilite (Wo 2.5–3En 71–73Fs 24–25), and augite (Wo 42–43En 42–43Fs 14–15), with accessory magnetite, minor ilmenite, apatite, sphene, and traces of zircon. The fractionation of calcic plagioclase+olivine+pyroxene±magnetite±ilmenite is responsible for the evolution of the parental magmas, which originated in approximately 10% melting of a mantle source contaminated by fluids from the oceanic subducted lithosphere, with sparse or no residual garnet. The gabbro rare earth element patterns are similar to those of the Antuco volcano (37°25′S) and other Quaternary basaltic rocks from the southern volcanic zone (CSVZ) of the Andes (37–41°5′S) emplaced in a relatively thin continental crust (≈30 km). Although the Cerro Nevazón granitoids share some geochemical signatures with the Paleogene Caicayén igneous units, also located in the northwestern region of Neuquén, they have different evolutionary styles, as expressed by the lack of amphibole fractionation and less Cs enrichment in the Caicayén rocks. With similar SiO 2 contents, the Upper Cretaceous–Paleocene Campana Mahuida subvolcanic rocks from the same region are richer in incompatible trace elements (K, Rb, Sr, Ba, Nb, La, Ce, Yb, Th, and U) than either the Nevazón or Caicayén units, which suggests that their parental magmas were contaminated with crustal material. The low Fe 2O 3/FeO ratios in the Nevazón igneous rocks, combined with the presence of ilmenite in most rocks and the low Mg/(Mg+Fe +2) ratios in mafic minerals, suggest that the Nevazón magmas evolved under more reduced conditions than did the Caicayén or Campana Mahuida magmas.

Geology of southeast Bohol, central Philippines: Accretion and sedimentation in a marginal basin

Recent field mapping has refined our understanding of the stratigraphy and geology of southeastern Bohol, which is composed of a Cretaceous basement complex subdivided into three distinct formations. The basal unit, a metamorphic complex named the Alicia Schist, is overthrust by the Cansiwang mélange, which is, in turn, structurally overlain by the Southeast Bohol Ophiolite Complex. The entire basement complex is overlain unconformably by a ∼2000 m thick sequence of Lower Miocene to Pleistocene carbonate and clastic sedimentary rocks and igneous units. Newly identified lithostratigraphic units in the area include the Cansiwang mélange, a tectonic mélange interpreted as an accretionary prism, and the Lumbog Volcaniclastic Member of the Lower Miocene Carmen Formation. The Cansiwang mélange is sandwiched between the ophiolite and the metamorphic complex, suggesting that the Alicia Schist was not formed in response to emplacement of the Southeast Bohol Ophiolite Complex. The accretionary prism beneath the ophiolite complex and the presence of boninites suggest that the Southeast Bohol Ophiolite Complex was emplaced in a forearc setting. The Southeast Bohol Ophiolite Complex formed during the Early Cretaceous in a suprasubduction zone environment related to a southeast‐facing arc (using present‐day geographical references). The accretion of this ophiolite complex was followed by a period of erosion and then later by extensive clastic and carbonate rock deposition (Carmen Formation, Sierra Bullones Limestone and Maribojoc Limestone). The Lumbog Volcaniclastic Member and Jagna Andesite document intermittent Tertiary volcanism in southeastern Bohol.