Mineralogy and pressure–temperature–time path of Cretaceous granulite gneisses, south-eastern San Gabriel Mountains, southern California

Mid-Cretaceous granulite gneisses crop out in a narrow belt in the Cucamonga region of the south-eastern foothills of the San Gabriel Mountains, southern California. Interlayered mafic granulites and pelitic, carbonate, calc-silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700–800°C. Regional deformation and formation of banded gneisses ceased by c. 108 Ma. although mafic-intermediate magmatism and high-grade metamorphism continued locally as late as c. 88 Ma. Garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near-isobaric cooling, but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo-chronological data. It is more likely that the entire terrane remained at granulite facies P–T conditions until 88 Ma, followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes. Uplift occurred between c. 88 and 78 Ma at rates of approximately 1–2 km Ma^-1. The geotectonic evolution of the Cucamonga granulites is similar to mid-Cretaceous high- P granulites in the Sierra Nevada and Salinian block of central California. Late Cretaceous uplift common to these granulites may provide an important tectonic link between dismembered Mesozoic batholithic terranes in the California Cordillera.     

Basalt-rhyolite volcanism by MORB-continental crust interaction: Nd,Sr-isotopic and geochemical evidence from southern San Joaquin Basin,California

The early miocene Tecuya volcanic center in the southern San Joaquin basin of California consists of flows and tuffs of basalt and rhyolite that erupted, closely spaced in time, in both submarine and subaerial conditions. The rhyolites are overlain by the basalts and constitute approximately 45% of a total of at least 180 km³ of the Tecuya volcanic rocks. The basalts have _Nd(t) values of +2 to +6 and (⁸⁷Sr/⁸⁶Sr)_i values between 0.7035 and 0.7052. These rocks show LREE enrichment [(La/Yb)_N =2.4–5.5; La=28–150 times chondrite] and higher Th/U, Th/Ta, Rb/Ta, Ba/Ta, Cs/Rb but lower K/Rb ratios than MORB. Combined major- and trace-element, and Sr–Nd isotopic data suggest the involvement of subcontinental lithosphere, depleted upper mantle source (MORB), and local continental crust in the basalt petrogenesis. _Nd(t) values in rhyolites vary from +1.5 to +3.7 while (⁸⁷Sr/⁸⁶Sr)_i ratios range from 0.7051 to 0.7064. The rhyolites display LREE enrichment [(La/Yb)_N=10; La=100 times chondrite] along with a distinct negative Eu anomaly (Eu/Eu^*=0.75) and depletion of Ti and P. Mixing relations in (⁸⁷/⁸⁶Sr)_i – _Nd(t) space among basalts, rhyolites, and local continental crust indicate that the Tecuya rhyolites were produced by assimilation of variable amounts of continental crust by MORB-related magmas and subcontinental lithosphere-derived melts. This conclusion is supported by the synchroneity of Tecuya volcanism at 22 Ma with interaction of a segment of the East Pacific Rise along the southern California margin. The Tecuya volcanic rocks thus provide an example for the generation of rhyolitic melts owing to crustal assimilation by basaltic melts during mid-oceanic ridge-induced magmatism along a continental margin.     

Metamorphic convergence of the upper and lower plates of the Vincent thrust, San Gabriel Mountains, southern California, USA

The Vincent thrust of the San Gabriel Mountains, southern California, separates eugeoclinal Pelona Schist from overlying Precambrian to Mesozoic igneous and metamorphic rocks of North American continental affinity. The thrust is generally considered to be synmetamorphic because of similarity in structural orientations and mineral assemblages between the Pelona Schist and mylonites at the base of the upper plate. In this study, compositions of calcic amphibole and plagioclase in the upper plate and structurally high Pelona Schist were compared to further test this interpretation. Amphibole in the schist is mostly actinolite to actinolitic hornblende with high Na/Al ratio, indicating relatively high-P/low-T metamorphism. Individual grains are zoned, with concentrations of both Na and Al decreasing from cores to rims. Premylonitic amphibole in the upper plate is hornblende, tschermakite and pargasite with compositions indicative of low- or medium-P metamorphism. During mylonitization, this amphibole was replaced by actinolite to actinolitic hornblende with a similar range of Na and Al as amphibole rims in the Pelona Schist, but with slightly lower Na/Al ratio. This is consistent with the decrease of Na/Al up-section previously noted within the Pelona Schist of this area, and is considered to be the result of an inverted thermal gradient during thrusting. Convergence of composition between schist and upper plate also occurs for K and Ti contents of amphibole and An content of plagioclase. These features provide strong evidence that mylonitization of the upper plate is closely related in space and time to metamorphism of the Pelona Schist and therefore that the Vincent thrust is a remnant of the primary fault along which the Pelona Schist and correlative units were subducted beneath North America. Nonetheless, very fine-scale differences in amphibole composition between the schist and upper plate may indicate that metamorphic re-equilibration could not quite keep pace with movement on the fault.     

Sm-Nd and Rb-Sr isotopic and geochemical systematics in Phanerozoic granulites from Fiordland,southwest New Zealand

Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70391±4. _Nd values (at 120 Ma) show a relatively wide range of from –0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of from 0.70380 to 0.70430 and _Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured _Nd(0) value of –10.2, ⁸⁷Sr/⁸⁶Sr ratio of 0.73679 and a T ^Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below 300° C by 100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher _Nd values (3.9 to 4.6) and slightly lower ⁸⁷Sr/ ⁸⁶Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (La_N/ Yb_N=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of _Nd values, but uniform initial ⁸⁷Sr/⁸⁶Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by 20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.     

Chemical and isotopic composition of the lower crust beneath the Meguma Lithotectonic Zone,Nova Scotia: evidence from granulite facies xenoliths

Granulite-facies xenoliths from an Upper Devonian lamprophyre dyke near Tangier, Nova Scotia, provide new information about the lower crust in the Meguma Zone. Two mineralogically and chemically distinet groups of xenoliths occur. Both groups contain quartz+feldspar+biotite+Fe–Ti oxides+sulfides. In addition, group A contains garnet+graphite±[aluminosilicates+spinel±sapphirine (hight Al₂O₃ subgroups A1 and A2)] or [clinopyroxene+sphene+apatite (high CaO sub-group A3)]. Group B has highly variable proportions of orthopyroxene (B1), clinopyroxene (B2), and amphibole (B3). Trace-element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other undepleted granulite-facies rocks. LowP-T sedimentary assemblages of quartz-chlorite-clay minerals-calcite-albite or paragonite can account for the compositions of group A xenoliths. Within group B, a high-MgO (MgO>13 wt%) subgroup with high transition-metal contents, and low-MgO (MgO<9 wt%) sub-groups with higher LIL (large-ion-lithophile) element contents exist. Although the rare-earth and high-field-strength elements indicate a tholeiitic or low-K calc-alkaline chemistry, the LIL elements are as high as those from high-K calc-alkaline volcanic are rocks. Isotopically, group A ranges from Nd_t=-2.56 to-0.80 and⁸⁷Sr/⁸⁶Sr _t =0.7046 to 0.7182 fort=370 Ma. For group B these values are +1.45 to +5.33 and 0.7028 to 0.7048, respectively. Model ages (T_CHUR) are correspondingly low and are tightly constrained (544±52 Ma). These young ages contrast with the middle Proterozoic Nd model ages of the overlying Meguma Zone low-grade flysch. This time-inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous Tangier lower crust (Avalon), and an older allochthonous Meguma Group upper crust.     

The lower crust beneath cratonic north-east Europe: isotopic constraints from garnet granulite xenoliths

Lower crustal garnet-bearing mafic granulite xenoliths from beneath the cratonic areas of NE Europe (NW Russia, Belarus, Finland) have unradiogenic ¹⁴³Nd/¹⁴⁴Nd ratios that differ strongly from those of xenoliths from beneath Phanerozoic regions of the European plate and worldwide, but closely resemble xenoliths from other cratonic regions of the world. Phanerozoic lower crustal xenoliths worldwide also show a very limited range of Pb isotope compositions whereas most cratonic lower crustal xenoliths have more varied but usually unradiogenic Pb isotope compositions, plotting to the left of the Geochron. However, many of the xenoliths from beneath NE Europe plot on the right-hand side of the Geochron and also have more radiogenic ²⁰⁸Pb/²⁰⁴Pb ratios. Thus, the lower crust of NE Europe shows characteristics of both cratonic lower crust (unradiogenic Nd isotopes) and Phanerozoic lower crust (radiogenic Pb isotopes). Its present-day low U/Pb and Th/Pb ratios indicate that it has been depleted in heat-producing elements, but the radiogenic Pb isotope ratios show that this depletion occurred relatively recently.     

Lithium isotopic evidence for subduction of the Indian lower crust beneath southern Tibet

Post-collisional K-rich volcanic rocks (KVRs) can provide an opportunity to constrain the architecture of the lithosphere and the mechanisms of plateau uplift. However, their petrogenesis and geodynamic setting remain in dispute. Lithium concentrations and isotopic compositions of 87 potassic, ultrapotassic and Mg-rich potassic volcanic rocks (PVRs, UPVs, and MPRs, respectively) in SW Tibet, along with new Pb–Sr–Nd isotope data and whole-rock analyses, are used to constrain their mantle source and genesis. These rocks are characterized by very similar δ⁷Li values: PVRs vary from −4.9‰ to +3.2‰, UPVs from −3.9‰ to +1.7‰, and MPRs from −1.2‰ to +3.5‰. They can be classified into two groups: Group I (19 out of 87 samples) with heavier δ⁷Li values (+1.0‰ to +3.5‰) similar to those reported for mid-ocean-ridge and ocean-island basalts (MORBs and OIBs, respectively), and Group II (68 out of 87 samples) with lighter values (−4.9‰ to +1.0‰) similar to those of Indian lower crust. These variable isotopic compositions may record the isotopic signature of the early-middle Miocene subcontinental lithospheric mantle (SCLM). This paper demonstrates the existence of isotopically light mantle domains beneath the Lhasa terrane, which were ascribed to the interaction with fluids/melts derived from the subducted Indian lower crust. The modeling curves of Indian lower crust with a metasomatized mantle composition fully account for compositional variations in the PVRs, UPVs, and MPRs. They were generated by the partial melting of SCLM, which was metasomatized by fluids/melts derived from the subducted Indian lower crust (ca. 4–14%, ca. 4–10%, and ca. 6–10% for the PVRs, UPVs, and MPRs, respectively). The Li isotopic data indicate that the Indian lower crust was subducted beneath the central Lhasa subterrane, and this sheds new light on the formation of the Tibet Plateau.     

Destruction of ancient lower crust through magma underplating beneath Jiaodong Peninsula, North China Craton: U-Pb and Hf isotopic evidence from granulite xenoliths

Zircons from granulite xenoliths entrained in a Late Cretaceous mafic dike in the Jiaodong Peninsula, North China Craton (NCC), show three distinct U-Pb age populations. Part of the old zircon grains yield discordant data that project to ages of about 2.4 to 2.5 Ga, a few grains indicate growth at about 2.0 Ga and a third group yield Cretaceous ages with peaks at 120 and 90 Ma. The oldest zircons give Hf T_DM model ages of 2.6-2.8 Ga. These results demonstrate the existence of original Archean lower crust in the Jiaodong region. Zircons of 2.0 Ga have similar Hf T_DM model ages as the Neoarchean-Paleoproterozoic grains, suggesting that these zircons were products of metamorphic recrystallization due to thermal event without juvenile input. Early Cretaceous zircons yield εHf(t) values of − 21 to − 12 and Late Cretaceous zircons large variable εHf(t) from + 4 to − 50. These data suggest that magmatic underplating occurred in the Neoarchean to Earliest Proterozoic lower crust of the NCC, both in the Early and Late Cretaceous. It is suggested that the Mesozoic magma underplating, which also provided the heat source for the voluminous Mesozoic magmatism in the NCC, significantly modified the composition of the Archean to Paleoproterozoic lower crust of the NCC.     

Continental crust and lithospheric mantle interaction beneath North China: isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean craton

Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.     

Geochemistry and petrogenesis of the marginal zone of the Mount Lowe Intrusion,central San Gabriel Mountains,California

The marginal zone of the Mount Lowe Intrusion (MLI) is composed of a mineralogically and geochemically distinctive suite of quartz poor, alkali-enriched monzodiorites and quartz monzodiorites. Low initial ⁸⁷Sr/⁸⁶Sr, Rb, and Rb/Sr coupled with high Ab/Or, K/Rb, Ba and Sr suggest that feldspathic continental crust did not contribute significantly to the magma. The inferred original magma composition is most consistent with experimental data for about 20% batch modal melting of eclogite or quartz eclogite of altered oceanic tholeiite composition. The high absolute Ba and Sr abundances require in addition an enriched component, perhaps derived from dehydration processes within a subducted slab. At the level of exposure, the range of observed compositions resulted from fractionation of hornblende, plagioclase, sphene, apatite and magnetite. Fractionation resulted in formation of a peraluminous, Mn — enriched residual liquid which crystallized garnet. The grandite — enriched composition of the garnet is inferred to be due to high pressure (ca. 6.7 kb), high crystallization, relative to typical plutonic garnets. The composition of the marginal zone of MLI has many chemical similarities to early Mesozoic plutonism in the Cordilleran continental margin magmatic arc of western North America.     

Accretion of juvenile crust at the Early Palaeozoic Antarctic margin of Gondwana: geochemical and geochronological evidence from granulite xenoliths

Geodynamic models for the Antarctic sector of the active Early Palaeozoic Palaeo-Pacific margin of Gondwana are based on the nature and age of the deep crust of the Robertson Bay terrane, the outermost lithotectonic unit of the margin. As this crustal block is covered with thick turbidite deposits, the only way to probe the deep crust is through the analysis of granulite xenoliths from Cenozoic scoria cones. Low-K felsic xenoliths yield the oldest (Middle Cambrian) laser-probe U–Pb ages on zircon areas with igneous growth zoning. This finding, along with the positive whole-rock ε_Nd(500Ma), suggests that these felsic rocks derived from a juvenile magma formed during the Early Palaeozoic Ross orogenic cycle. Mafic xenoliths have geochemical-isotopic compositions similar to those of modern primitive island arcs, suggesting the involvement of subducted oceanic crust in their magma genesis and accretion of juvenile crust at the Antarctic margin of Gondwana.     

苏鲁造山带胶南区段片麻岩原岩的成因:地球化学及Nd同位素证据

出露于苏鲁造山带胶南区段的片麻岩主要包括三种类型黑云斜长片麻岩、二长花岗片麻岩和A型花岗质片麻岩,地球化学判别结果表明它们的原岩均为火成岩.不同类型片麻岩的地球化学特征各不相同,反映它们的原岩具有不同的形成与演化历史.其中黑云斜长片麻岩以负Eu异常弱,甚至出现明显的正异常(δEu=0.60～1.45)、强的Ba正异常、无明显的Sr异常为特征而明显不同于其它类型的片麻岩.其地球化学的总体特征类似于扬子克拉通北缘新元古代双峰式火山岩的酸性端元;与黑云斜长片麻岩类似,二长花岗片麻岩中轻、重稀土元素之间的分馏程度也较强(LaN/YbN=6.3～17.2),但以强的负Eu异常(δEu=0.27～0.54)、强的Sr负异常和弱的Ba负异常为特征明显不同于黑云斜长片麻岩;A型花岗片麻岩的稀土模式及蛛网图的形态类似于二长花岗片麻岩,所不同的是前者Nb、Ta负异常相对较弱.区内几种类型片麻岩的εNd(t)值差别不大,反映它们的原岩之间可能有一定的成因联系.它们的Nd同位素模式年龄TDM都集中在2.0Ga左右,表明它们的源区主要为早元古代的地壳物质,且岩性比较均一.推测二长花岗片麻岩与黑云斜长片麻岩的原岩是同源异相,而A型花岗岩是在大规模Ⅰ型花岗质岩浆形成后,由脱水的紫苏辉石质残留下地壳物质再次发生部分熔融形成的.     

Experimental and geochemical evidence for derivation of the El Capitan Granite,California, by partial melting of hydrous gabbroic lower crust

Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850°C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635–661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486–1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening.     

Sources of Phanerozoic granitoids in the transect Bayanhongor–Ulaan Baatar, Mongolia: geochemical and Nd isotopic evidence, and implications for Phanerozoic crustal growth

The Central Asian Orogenic Belt (CAOB) is renowned for massive generation of juvenile crust in the Phanerozoic. Mongolia is the heartland of the CAOB and it has been subject to numerous investigations, particularly in metallogenesis and tectonic evolution. We present new petrographic, geochemical and Sr–Nd isotopic analyses on Phanerozoic granitoids emplaced in west-central Mongolia. The data are used to delineate their source characteristics and to discuss implications for the Phanerozoic crustal growth in Central Asia. Our samples come from a transect from Bayanhongor to Ulaan Baatar, including three tectonic units: the Baydrag cratonic block (late Archean to middle Proterozoic), the Eo-Cambrian Bayanhongor ophiolite complex and the Hangay–Hentey Basin of controversial origin. The intrusive granitoids have ages ranging from ca. 540 to 120 Ma. The majority of the samples are slightly peraluminous and can be classified as granite (s.s.), including monzogranite, syenogranite and alkali feldspar granite. Most of the rocks have initial ⁸⁷Sr/⁸⁶Sr ratios between 0.705 and 0.707. Late Paleozoic to Mesozoic granitoids (≤250 Ma) are characterized by near-zero _Nd(T) values (0 to −2), whereas older granitoids show lower _Nd(T) values (−1.5 to −7). The data confirm the earlier observation of Kovalenko et al. [Geochemistry International 34 (1996) 628] who showed that granitoids emplaced outside of the Pre-Riphean basement rocks are characterized by juvenile positive _Nd(T) values, whereas those within the Pre-Riphean domain and the Baydrag cratonic block, as for the present case, show a significant effect of ‘contamination’ by Precambrian basement rocks. Nevertheless, mass balance calculation suggests that the granitoids were derived from sources composed of at least 80% juvenile mantle-derived component. Despite our small set of new data, the present study reinforces the general scenario of massive juvenile crust production in the CAOB with limited influence of old microcontinents in the genesis of Phanerozoic granitoids.     

Nd-Sr isotopic and geochemical systematics in Cambrian boninites and tholeiites from Victoria,Australia

Rocks with boninitic affinities have been recognised in a number of ophiolites, including the Cambrian Heathcote and Mt Wellington Greenstone Belts of Victoria. Boninites and high-Mg andesites from the Heathcote Greenstone Belt show a restricted range of initial _Nd values of between +3.3 to +5.8. Extremely refractory boninites from the Mt Wellington Greenstone Belt have _Nd ranging from +1.3 to –9. Ti/Zr is positively correlated with Sm/Nd with the Heathcote lavas generally possessing greater depletion of Ti and enrichment of Zr relative to the middle and heavy REE with increasing LREE/HREE. These data are consistent with the generation of boninites by partial melting of refractory peridotite following invasion by LREE- and Zr-enriched, low _Nd fluids. Tholeiites overlying the boninites in both greenstone belts have flat REE patterns and _Nd+5, lower than that anticipated for lavas derived from depleted MORB source reservoirs in the Cambrian, suggesting that their source was also contaminated by a LREE-enriched, low _Nd component similar to that involved in the generation of the Howqua boninites. The added components have characteristics compatible with their derivation from subducted altered oceanic crust and/or from wet subducted sediments. The identification of boninites and other low-Ti lavas in the Victorian greenstone belts is strong evidence for island arc development in southeastern Australia during the Lower Cambrian and provides further support for a subduction-related origin for many ophiolites.     

Geochronological, isotopic, and geochemical data from Permo-Triassic granitic gneisses and granitoids of the Colombian Central Andes

New U–Pb SHRIMP ages in zircon, Ar–Ar ages in micas and amphiboles, Nd–Sr isotopes, and major and REE geochemical analyses in granitic gneisses and granitic stocks of the Central Cordillera of Colombia indicate the presence of a collisional orogeny in Permo-Triassic times in the Northern Andes related to the construction of the Pangea supercontinent. The collision is recorded by metamorphic U–Pb SHRIMP ages in inherited zircons around 280 Ma and magmatic U–Pb SHRIMP ages in neoformed zircons around 250 Ma within syntectonic crustal granitic gneisses. Magmatic U–Pb SHRIMP and Ar–Ar Triassic ages around 228 Ma in granitic stocks indicate the presence of late tectonic magmatism related to orogenic collapse and the beginning of the breakup of the supercontinent. During this period, the Central Cordillera of Colombia would have been located between the southern United States and northern Venezuela, in the leading edge of the Gondwana supercontinent.     

The evolution of a silicic magma system: isotopic and chemical evidence from the Woods Mountains volcanic center,eastern California

The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have _Nd values of -6.2 to -7.5 and initial ⁸⁷Sr/⁸⁶Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the _Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the ⁸⁷Sr/⁸⁶Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO₂ contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.     

青藏高原北羌塘榴辉岩质下地质及富集型地幔源区——来自新生代火山岩的岩石地球化学证据

利用岩石地球化学的方法，研究了北羌塘新生代火山岩，结果表明，北羌塘第三纪火山岩可区分为碱性（钾玄岩质）和高钾钙碱两套不同的火山岩组合，它们分别起源于一个特殊的，不均一的富集型上地幔和一个加厚陆壳的榴辉岩质下地壳，由于青藏陆块之下软流层物质的上涌而形成幔源碱性岩浆活动，而幔源岩浆在Mobo面的底侵作用又为下地壳中酸性高钾钙碱系列火山岩的起源提供了热动力条件，该区两套不同系列和源区类型的火山岩正是在这种特殊的构造环境中形成。     

Crustal contamination as an indicator of the extent of early Archaean continental crust: Pb isotopic evidence from the late Archaean gneisses of West Greenland

Ph isotopic analyses are reported for 119 samples of late Archaean (ca. 3000-2800 Myr) calc alkaline orthogneisses and associated anorthosites from southern West Greenland. Over most of the area. $\text{Pb}\text{Pb}$ whole rock isotope systematics indicate derivation of the magmatic precursors of the gneisses and anorthosites from a source region with a typically mantle-type $\text{U}\text{Pb}$ ratio (μ₁ value of 7.5) at. or shortly before, ca. 3000-2800 Myr ago. In contrast, in the Godthaabsfjord region, late Archaean Nûk gneisses and associated anorthosites were emplaced into or through early Archaean (ca. 3700 Myr) Amîtsoq gneisses, and crystallised with variable proportions of two isotopically distinct types of Pb which commenced their respective crustal developments at ca. 3000-2800 Myr and at ca. 3700 Myr ago. Isotopic and other geochemical constraints demonstrate that Nûk gneisses and their temporal equivalents were not produced by reworking or melting of Amîtsoq gneisses. Mixing of early and late Archaean Pb results from contamination of the magmatic precursors of Nûk gneisses and anorthosites (characterised by mantle-type Pb at time of emplacement) with ancient, unradiogenic Pb derived from ca. 3700 Myr-old Amîtsoq-type continental crust invaded by the Nûk magmas. The contaminant is considered to be a trace-element enriched fluid phase released from dehydrating older continental crust during progressive burial and heating by emplacement of calc alkaline magmas in the late Archaean ‘accretion differentiation superevent’. This was followed by mixing of the released fluids with younger Nûk magmas.Pb isotopic compositions of late Archaean gneisses and anorthosites outside the Godthaabsfjord region provide no evidence for the presence of early Archaean Amîtsoq-type continental crust in southern West Greenland in areas more than a few tens of km outside the known outcrop of Amîtsoq gneisses. We suggest that early Archaean crust does not exist at depth in late Archaean areas with undisturbed Pb-isotope systematics, either in Greenland or elsewhere in the North Atlantic craton.Pb-isotope evidence for crust magma interaction, involving selective extraction of certain trace elements by a fluid phase from wall rock and subsequent mixing between magma and contaminant fluid, provides a powerful tool for detection, sub-surface ‘mapping’, and geochronological and geochemical characterisation of deep, ancient continental crust.