Material records for Mesozoic destruction of the North China Craton by subduction of the Paleo-Pacific slab

It is well known that the destruction of the North China Carton(NCC) is closely related to subduction of the PaleoPacific slab, but materials recording such subduction has not been identified at the peak time of decratonization. This paper presents data of whole-rock major and trace elements and Sr-Nd-Hf isotopes and zircon U-Pb ages and Hf-O isotopes for Mesozoic volcanic rocks from the Liaodong-Jinan region in the northeastern NCC, in order to trace the subduction-related materials in their source and origin. The Mesozoic volcanic rocks in the Liaodong-Jinan region are mainly composed of two series of rocks, including alkaline basaltic trachyandesite, trachyandesite and trachyte, and subalkaline trachyandesite and andesite. Zircon U-Pb dating yields eruption ages of 129–124 Ma for these rocks. The Early Cretaceous volcanic rocks are all enriched in LILEs(such as Rb, Sr, Ba and Th) and LREEs, depleted in HFSEs(such as Nb, Ta and Ti), indicating that they were originated from mantle sources that had been modified by subducted crustal materials. However, they have relatively heterogeneous and variable isotopic compositions. The alkaline basaltic trachyandesite, trachyandesite and trachyte have enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions and mantle-like δ~(18)O values, suggesting that they were derived from low-degree partial melting of an isotopically enriched lithospheric mantle source. In contrast, the subalkaline trachyandesite and andesite have relatively depleted isotopic compositions with zircon ε_(Hf)(t) values up to +5.2 and heavy zircon O isotopic compositions with δ~(18)O values of +8.1‰ to +9.0‰, indicating that they were originated from a lithospheric mantle source that had been metasomatized by melts/fluids derived from the recycled low-T altered oceanic basalt. All of these geochemical features suggest that the Early Cretaceous volcanic rocks in the Liaodong-Jinan region would result from mixing of mafic magmas with different compositions. Such magmas were originated from the enriched lithospheric mantle and the young metasomatized mantle, respectively, with variable extents of enrichment and depletion in trace elements, radiogenic isotopes and O isotopes. Importantly, the identification of the low-T altered oceanic crust component in the origin of Early Cretaceous volcanic rocks by the zircon Hf-O isotopes provides affirmative isotopic evidence and direct material records for Mesozoic subduction of the Paleo-Pacific slab that induced decratonization of the North China Craton.     

Age and generation of Fogang granite batholith and Wushi diorite-hornblende gabbro body

Fogang granitic batholith, the largest Late Mesozoic batholith in the Nanling region, has an exposure area of ca. 6000 km2. Wushi diorite-hornblende gabbro body is situated at the northeast part of the ba- tholith. Both the granitic batholith main body and the diorite-hornblende gabbro body belong to high-K calc alkaline series. Compared with the granitic main body, the Wushi body has lower Si (49%―55%), higher Fe, Mg, Ca, lower REE, less depletion of Eu, Ba, P, Ti, and obvious depletion of Zr, Hf. Zircon LA-ICP-MS dating and the mineral-whole rock isochron dating reveal that Fogang granitic main body and Wushi body were generated simultaneously at ca. 160 Ma. The Fogang granitic main body has high (87Sr/86Sr)i ratios (0.70871―0.71570) and low εNd(t) values (?5.11―?8.93), suggesting the origins of the granitic rocks from crustal materials. Their Nd two-stage model ages range from 1.37―1.68 Ga. The Sr and Nd isotopic compositions and the Nd model ages of the granitic rocks may suggest that the giant Fogang granitic main body was generated from a heterogeneous source, with participation of mantle component. Wushi diorite-hornblende gabbro is an unusual intermediate-basic magmatic rock series, with high (87Sr/86Sr)i ratios (0.71256―0.71318) and low εNd(t) values (?7.32―?7.92), which was possibly formed through mixing between the mantle-derived juvenile basaltic magma and the magma produced by the dehydration melting of lower crustal basaltic rocks.     

Petrogenetic differences between the Middle-Late Jurassic Cu-Pb-Zn-bearing and W-bearing granites in the Nanling Range,South China: A case study of the Tongshanling and Weijia deposits in southern Hunan Province

The Middle-Late Jurassic Cu-Pb-Zn-bearing and W-bearing granites in the Nanling Range have distinctly different mineralogical and geochemical signatures. The Cu-Pb-Zn-bearing granites are dominated by metaluminous amphibole-bearing granodiorites, which have higher CaO/(Na2O+K2O) ratios, light/heavy rare earth element(LREE/HREE) ratios, and δEu values,lower Rb/Sr ratios, and weak Ba, Sr, P, and Ti depletions, exhibiting low degrees of fractionation. The W-bearing granites are highly differentiated and peraluminous, and they have lower CaO/(Na2O+K2O) ratios, LREE/HREE ratios, and δEu values,higher Rb/Sr ratios, and strong Ba, Sr, P, and Ti depletions. The Cu-Pb-Zn-bearing granites were formed predominantly between155.2 and 167.0 Ma with a peak value of 160.6 Ma, whereas the W-bearing granites were formed mainly from 151.1 to 161.8Ma with a peak value of 155.5 Ma. There is a time gap of about 5 Ma between the two different types of ore-bearing granites.Based on detailed geochronological and geochemical studies of both the Tongshanling Cu-Pb-Zn-bearing and Weijia W-bearing granites in southern Hunan Province and combined with the other Middle-Late Jurassic Cu-Pb-Zn-bearing and W-bearing granites in the Nanling Range, a genetic model of the two different types of ore-bearing granites has been proposed. Asthenosphere upwelling and basaltic magma underplating were induced by the subduction of the palaeo-Pacific plate. The underplated basaltic magmas provided heat to cause a partial melting of the mafic amphibolitic basement in the lower crust, resulting in the formation of Cu-Pb-Zn mineralization related granodioritic magmas. With the development of basaltic magma underplating,the muscovite-rich metasedimentary basement in the upper-middle crust was partially melted to generate W-bearing granitic magmas. The compositional difference of granite sources accounted for the metallogenic specialization, and the non-simultaneous partial melting of one source followed by the other brought about a time gap of about 5 Ma between the Cu-Pb-Zn-bearing and W-bearing granites.     

Multiple-aged granitoids and related tungsten-tin mineralization in the Nanling Range, South China

The Nanling metallogenic belt in South China is characterized by well-developed tungsten-tin mineralization related to multiple-aged granitoids. This belt is one of the 5 key prospecting and exploration areas among the 19 important metallogenic targets in China. Important progress has been made in recent years in understanding the Nanling granitoids and associated mineralization, and this paper introduces the latest major findings as follows: (1) there exists a series of Caledonian, Indosinian, and Yanshanian W-Sn-bearing granites; (2) the Sn-bearing Yanshanian granites in the Nanling Range form an NE-SW trending aluminous A-type granite belt that stretches over 350 km. The granites typically belong to the magnetite series, and dioritic micro-granular enclaves with mingling features are very common; (3) the Early Yanshanian Sn- and W-bearing granites possess different petrological and geochemical features to each other: most Sn-bearing granites are metaluminous to weakly peraluminous biotite (hornblende) granites, with zircon ?_Hf(t) values of ca. ?2 to ?8, whereas most W-bearing granites are peraluminous two-mica granites or muscovite granites with ?_Hf(t) values of ca. ?8 to ?12; (4) based on the petrology and geochemistry of the W-Sn-bearing granites, mineralogical studies have shown that common minerals such as titanite, magnetite, and biotite may be used as indicators for discriminating the mineralizing potential of the Sn-bearing granites. Similarly, W-bearing minerals such as wolframite may indicate the mineralizing potential of the W-bearing granites. Future studies should be focused on examining the internal relationships between the multiple-aged granites in composite bodies, the metallogenic peculiarities of multiple-aged W-Sn-bearing granites, the links between melt evolution and highly evolved ore-bearing felsic dykes, and the connections between granite domes and mineralization.     

Crust evolution in Southeast China:evidence from Nd model ages of granitoids

Nd isotopic compositions of 58 granitoids in South China have been reported in this paper.These data together with other published data reveal that granites with Nd model ages (tDM) greater than 1.8 Ga are distributed mainly in three areas:southwestern Zhejiang-northwestern Fujian,two sides of the Wuyi Mountain and Wanyangshan-Zhuguangshan.These granites are believed to be derived from partial melting of old crust in these areas.The Mesozoic granites with tDM＜1.6 Ga are distributed in three zones:the Gangang structural zone,Nanling latitudinal structural zone and Fujian-Zhejiang coastal zone.These zones may have been an extensional tectonic setting and mantle-derived components or magmas may have been involved to different extents in the granite formation.Based on Nd model ages of granites and published chronological data of mafic and ultramafic rocks,it is believed that the crust in South China experienced episodic accretions,among which the early-middle Proterozoic is the most important period of crustal accretion.     

Petrogenesis of mafic enclaves from the north Taihang Yanshanian intermediate to felsic plutons: Evidence from petrological,geochemical, and zircon Hf-O isotopic data

We report the zircon Hf-O isotopic data for mafic enclaves from the Taihang Yanshanian intermediate to felsic plutons, and use them, along with the petrological, whole-rock chemical and Nd-Sr isotopic data, to reveal the petrogenesis of mafic enclaves. Mafic enclaves show magmatic textures and are finer-grained than host rocks. In places they are highly elongated due to stretching within the partially crystallized, convective felsic magma, but show no solid-state deformation. These data suggest that mafic e...     

Geochronological Significance of the Post-Orogenic Mafic- Ultramafic Rocks in the Hongqiling Area of Jilin Province, Northeast China

INTRODUCTIONThe mafic-ultramafic complexesinthe Hongqilingarea were emplacedintothe metamorphic rocksof the Hulan Group. Age determination of the intrusion and metamorphism of the Hulan Groupmetamorphic rocks is crucial for the study of petrogenesis and evolution, orogenesis and itsdevelopment of the region. However ,so far it has been difficult to determine the geochronology ofmafic-ultramafic rocks inthe area ,thusthe age obtainedfromprevious data hadto be used.Inrecentyears ,withthe …     

Geochemical characteristics and Sr‐Nd‐Hf isotope compositions of Late Triassic post‐collisional A‐type granites in Sarudik,SW Sumatra,Indonesia

Late Triassic A‐type granites are identified in this study in Sarudik, SW Sumatra. We present new data on zircon U–Pb geochronology, whole‐rock major and trace elements and Sr‐Nd‐Hf isotope geochemistry, aiming to study their petrogenesis and tectonic implications. LA‐ICP‐MS U–Pb dating of zircon separated from one biotite monzogranite sample yields a concordia age of 222.6 ±1.0 Ma, indicating a Late Triassic magmatic event. The studied granites are classified as weakly peralumious, high‐K calc‐alkaline granites. They exhibit high SiO₂, K₂O + Na₂O, FeO/(FeO + MgO) and Ga/Al ratios and low Al₂O₃, CaO, MgO, P₂O₅ and TiO₂ contents, with enrichment of Rb, Th and U and depletion of Ba, Sr, P and Eu, showing the features of A‐type granites. The granites have zircon ε_Hf(t) values from ?4.6 to ?0.4 and whole‐rock ε_Nd(t) values from ?5.51 to ?4.98, with Mesoproterozoic T_DM2 ages (1278–1544 Ma) for both Hf and Nd isotopes. Geochemical and isotopic data suggest that the source of these A‐type granites is the Mesoproterozoic continental crust, without significant incorporation of mantle‐derived component, and their formation is controlled by subsequent fractional crystallization. The Sarudik A‐type granites are further assigned to A2‐type formed in post‐collisional environment. Combined with previous knowledge on the western SE Asia tectonic evolution, we conclude that the formation of the Late Triassic A‐type granites is related to the post‐collisional extension induced by the crustal thickening, gravitational collapse, and asthenosphere upwelling following the collision between the Sibumasu and the East Malaya Block.     

Chronological and geochemical studies of granite and enclave in Baimashan pluton,Hunan, South China

Zircon LA-ICP-MS U-Pb dating reveals that the Baimashan Pluton is composed mainly of late Indosinian (204.5±2.8 Ma-209.2±3.8 Ma) biotite granodiorites/monzonitic granites (LIGs) and early Yanshanian (176.7±1.7 Ma) two-micas monzonitic granites (EYGs), and the coeval (203.2±4.5 Ma-205.1±3.9 Ma) mafic microgranular enclaves (MMEs) are generally found in the former. In addition, the ages of cores within zircons from LIGs and MMEs ranging from 221.4±4.0 Ma to 226.5±4.1Ma provide evidence of multistage magma intrusion during Indosinian in the study area. Measured 3010±20.6 Ma of inherited zircon age suggests that there may be recycling Archaean curstal material in existence in this area. LIGs and EYGs share some similar geochemical features: subalkaline and peraluminous granites, enrichment of Th, U, K, Ta, Zr, Hf and LREE but depletion of Ba, Nb, P, Ti and Eu, low εNd(t) values but high (87Sr/86Sr)i ratios, and old T2DM (ca. 1.9-2.0 Ga). The behaviors of incompatible elements and REE are mainly dominated by fractional crystallization of plagioclase, K-feldspar, ilmenite and apatite, but that of Sr isotope mainly controlled by EC-AFC. They are crust-sourced and derived from partial melting of paleo-Proterozoic metagreywackes and related to biotite dehydration melting. LIGs are formed in post-collisional tectonic setting as crustal local extension and thinning during late Indosinian. But EYGs may be evolved products of congeneric granitic magma with LIGs formed in late Indoinian, which were emplaced again when crust underwent extensive thinning and extension in post-orogenic tectonic setting during Yanshanian in SC after undergoing EC-AFC. MMEs should be cognate enclaves and derived from liquid immiscibility of host magma.     

Tracking the lithium isotopic evolution of the mantle using carbonatites

Carbonatites are mantle-derived, intraplate magmas that provide a means of documenting isotopic variations of the Earth's mantle through time. To investigate the secular Li isotopic evolution of the mantle and to test whether Li isotopes document systematic recycling of material processed at or near the Earth's surface into the mantle, we analyzed the Li isotopic compositions of carbonatites and spatially associated mafic silicate rocks. The Li isotopic compositions of Archean (2.7 Ga) to Recent carbonatites (δ⁷Li = 4.1 ± 1.3 (n = 23, 1σ)) overlap the range typical for modern mantle-derived rocks, and do not change with time, despite ongoing crustal recycling. Thus, the average Li isotopic composition of recycled crustal components has not deviated greatly from the mantle value (~ + 4) and/or Li diffusion is sufficiently fast to attenuate significant heterogeneities over timescales of 10⁸ years. Modeling of Li diffusion at mantle temperatures suggests that limited δ⁷Li variation in the mantle through time reflects the more effective homogenization of Li in the mantle compared to radiogenic isotope systems. The real (but limited) variations in δ⁷Li that exist in modern mantle-derived magmas as well as carbonatites studied here may reflect isotopic fractionation associated with shallow-level processes, such as crustal assimilation and diffusive isotopic fractionation in magmatic systems, with some of the scatter possibly related to low-temperature alteration.     

Extreme Nd isotope homogeneity in a large rhyolitic province: the Estérel massif, southeast France

 Previous detailed studies of large rhyolite bodies propose that their elemental and isotopic characteristics were largely acquired in shallow crustal magma chambers. This model explains the common chemical and isotopic zonations of large volumes of rhyolites as well as the less common chemical and isotopic homogeneity of such bodies. We report an intermediate situation (the Estérel massif, southeast France) in which chemical variations contrast with Nd-isotope homogeneity. We thus infer that, in this case, large volumes of rhyolite resided for enough time in shallow magma chambers to develop chemical zonations through differentiation, but this process was not accompanied by crustal assimilation. The subordinate amount of mafic rocks cropping out in the Estérel probably evolved from basalt to trachyte through assimilation and fractional crystallization. The relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust show that it cannot have been generated by partial melting of exposed basement rocks. Several geological similarities with large rhyolitic provinces could suggest that the rhyolite was purely mantle derived or, alternatively, generated by partial melting of an ad hoc crustal component. However, mineralogical, geochemical, and geodynamic connections between the Estérel rhyolite and the hypersolvus anorogenic granites of Corsica, as well as the extreme Nd-isotope homogeneity of the rhyolite, lead us to propose that the rhyolite was generated by mixing between mantle-derived magmas and a mafic lower crust. This scenario accounts for the relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust. The good Nd-isotope homogeneity observed in the rhyolite implies that the mixing process, which occurred in the deep crust, was complete and provided a shallow magma chamber with isotopically and probably chemically homogeneous magmas. Received: 5 December 1997 / Accepted: 16 June 1998     

Strongly peraluminous granites of Mesozoic in Eastern Nanling Range,southern China: Petrogenesis and implications for tectonics

The strongly peraluminous granites (SPGs) of Eastern Nanling Range (ENR) are a characteristic of all bearing highly aluminous minerals, such as muscovite±AI-rich biotite±tourmaline±garnet, and lack of cordierite. In respect of petrography, geochemistry, Nd isotope, and single grain zircon U-Pb dating, the representative granite bodies of them are studied. The research shows that these granites were emplaced in two stages, namely 228-225 Ma BP and J2-3 159-156 Ma BP, belonging to Indosinian and early Yanshanian periods, respectively, and they have low εNd(t) values (-10.6--11.1), high A/CNK, Rb/Sr ratios and tDM values (1887-1817 Ma), and REE's tetrad effect (TE1,3=1.13-1.34). In comparison with related geology, petrology and chronology of granites in adjacent regions, it is suggested that Indosinian SPGs of ENR formed in the circumstance of post-collisional extension 20 Ma after the major collision of Indosinian Movement (258-243 Ma BP) in Indo-China Peninsula, and early Yanshanian SPGs formed in the     

Mineralogical feature and geological significance of muscovites from the Longyuanba Indosinian and Yanshannian two-mica granites in the eastern Nanling Range

Emplacement P-T condition estimations using granites are important for understanding metamorphic and erosional processes of orogenic belt.Granites are widespread in South China and a majority of them are peraluminous.Particularly,over 91%of the Indosinian granites exposed in the region are peraluminous in composition.It is extremely hard to determine the pressure of intrusion of these peraluminous granites due to the absence of amphibole,a good mineral barometer commonly identified in metaluminous granites.Muscovite is a common mineral in peraluminous granites,certain kind of it could be used as a mineral barometer to constrain the emplacement pressure of peraluminous granites.In this paper,results of petrographic and geochemical studies of muscovites from the Indosinian and early Yanshanian two-mica granites at the Longyuanba in the eastern Nanling Range are reported.Based on petrographic studies,the primary muscovite can be discriminated from the secondary muscovites.Muscovites from the Indosinian two-mica granites are enriched in Ti,Al,Mg,and Na,and depleted in Fe and Mn.Geochemically,these muscovites were considered as primary,whereas those from the Yanshanian two-mica granites fall into the area of secondary muscovite on discrimination diagrams.Barometer estimations show that pressures calculated for primary muscovites are accurate,but those calculated for secondary muscovites are overestimated.The average pressure of emplacement of the Longyuanba Indosinian two-mica granites is 5.9 kbar,corresponding to~19 km in depth,suggesting that the Indosinian granitic magmas were probably generated by partial melting of a thickened crust root in a compressional tectonic setting.     

Postcollisional mantle-derived magmatism, underplating and implications for basement of the Junggar Basin

The late Paleozoic postcollisional granitoids, mafic-ultramafic complexes, and volcanic rocks are extensively distributed around the Junggar Basin; they are generally characterized by positive ε_Nd(t) values, implying that the magmas were mantle-derived and contaminated with crustal materials to some extents. The emplacement of mantle-derived magmas and their differentiates in the upper crust is the expression of deep geological processes at shallow level, while much more mantle-derived magmas were underplated in the lower crust and the region near the crust-mantle boundary, being component part of basement of the Junggar Basin. The postcollisional mafic-ultramafic complexes would not be generated by re-melting of residual oceanic crust, which was considered as the basement of the Junggar Basin, unless very high degrees of partial melting occurred. Even if old continental crust had been present before collision, it would have been strongly modified by the mantle-derived magma underplating. This interpretation is compatible with the existing geophysical data. Project supported by the National Natural Science Foundation of China (Grants Nos. 4900031 and 49272103)     

Sr and Nd isotope geochemistry of coexisting alkaline magma series,Cantal, Massif Central,France

Sr and Nd isotope analyses are presented for Tertiary continental alkaline volcanics from Cantal, Massif Central, France. The volcanics belong to two main magma series, silica-saturated and silica-undersaturated (with rare nephelinites). Trace element and isotopic data indicate a common source for the basic parental magmas of both major series; the nephelinites in contrast must have been derived from a mantle source which is isotopically and chemically distinct from that which gave rise to the basalts and basanites.⁸⁷Sr/⁸⁶Sr initial ratios range from 0.7034 to 0.7056 in the main magma series (excluding rhyolites) and¹⁴³Nd/¹⁴⁴Nd ratios vary between 0.512927 and 0.512669; both are correlated with increasing SiO₂ in the lavas. The data can be explained by a model of crustal contamination linked with fractional crystallisation. This indicates that crustal magma chambers are the sites of differentiation since only rarely do evolved magmas not show a crustal isotopic signature and conversely basic magmas have primitive isotopic ratios unless they contain obvious crustal-derived xenocrysts. Potential contaminants include lower crustal granulites or partial melts of upper crustal units. Equal amounts of contamination are required for both magma series, refuting hypotheses of selective contamination of the silica-saturated series.The isotopic characteristics of the apparently primary nephelinite lavas demonstrates widespread heterogeneity in the mantle beneath Cantal. Some rhyolites, previously thought to be extremely contaminated or to be crustally derived, are shown to have undergone post-emplacement hydrothermal alteration.     

Inherited isotope systems and the source region pre-history of early Caledonian granites in the Dalradian Series of Scotland

RbSr and UPb isotope analyses are reported for two pre-metamorphic Caledonian granites which intrude Dalradian rocks in the Central Highlands of Scotland. These data indicate that the origin of the granitic magmas involved partial fusion of old crustal material.UPb systems of zircon size and magnetic fractions from the Ben Vuirich granite are strongly discordant. However, U/Pb isotopic ratios precisely define a chord which intersects concordia at 514_?7⁺⁶ m.y. and 1316_?25⁺²⁶ m.y. Geological constraints suggest that the lower intersection records the post-F₂, pre-M₃ emplacement age of the granite. The upper intersection reflects the presence of old zircon xenocrysts incorporated into the granite magma without complete isotopic resetting. The ultimate source of these xenocrysts is probably a metamorphic basement complex which formed about 1320 m.y. ago, but the immediate source region of the granites could have been Dalradian sediments derived therefrom.RbSr whole-rock systems of the Ben Vuirich granite are also strongly discordant, although 8 out of 13 data points scatter about an “errorchron” of 564 ± 24 m.y. with an initial⁸⁷Sr/⁸⁶Sr ratio of about 0.716. This is interpreted as a spurious result due to incomplete homogenization of Sr isotopes in the source region during partial fusion. Initial⁸⁷Sr/⁸⁶Sr ratios at the time of emplacement indicated by the zircon data ranged from 0.7173 to 0.7191. Whole-rock samples from the Dunfallandy Hill granite have Rb/Sr ratios 2–3 times higher than those from Ben Vuirich and define a reasonably good isochron age of 491 ± 15 m.y. with an initial⁸⁷Sr/⁸⁶Sr of 0.7185 ± 0.0008. This may date granite emplacement or subsequent resetting of the high Rb/Sr rocks during Caledonian metamorphism. RbSr systematics indicate that the crustal source regions of these and other Caledonian granites separated from the upper mantle at least ca. 800 m.y. ago and probably ca. 1300 m.y. ago, thus confirming the interpretation of the upper intersection age of the zircon UPb data.     

Crustal accretion and reworking processes of micro-continental massifs within orogenic belt: A case study of the Erguna Massif,NE China

This paper summarizes the geochronological, geochemical and zircon Hf isotopic data for Mesozoic granitoids within the Erguna Massif, NE China, and discusses the spatial-temporal variation of zircon Hf isotopic compositions, with the aim of constraining the accretion and reworking processes of continental crust within the Erguna Massif, and shedding light on the crustal evolution of the eastern segment of the Central Asian Orogenic Belt. Based on the zircon U-Pb dating results, the Mesozoic granitic magmatisms within the Erguna Massif can be subdivided into five stages: Early-Middle Triassic(249–237 Ma), Late Triassic(229–201 Ma), Early-Middle Jurassic(199–171 Ma), Late Jurassic(155–149 Ma), and Early Cretaceous(145–125 Ma).The Triassic to Early-Middle Jurassic granitoids are mainly I-type granites and minor adakitic rocks, whereas the Late Jurassic to Early Cretaceous granitoids are mainly A-type granites. This change in magmatism is consistent with the southward subduction of the Mongol-Okhotsk oceanic plate and subsequent collision and crustal thickening, followed by post-collision extension. Zircon Hf isotopic data indicate that crustal accretion of the Erguna Massif occurred in the Mesoproterozoic and Neoproterozoic. ZirconεHf(t) values increase gradually over time, whereas two-stage model(TDM2) ages decrease throughout the Mesozoic. The latter result indicates a change in the source of granitic magmas from the melting of ancient crust to more juvenile crust. Zircon εHf(t)values also exhibit spatial variations, with values decreasing northwards, whereas TDM2 ages increase. This pattern suggests that,moving from south to north, there is an increasing component of ancient crustal material within the lower continental crust of the Erguna Massif. Even if at the same latitude, the zircon Hf isotopic compositions are also inconsistent. These results reveal lateral and vertical heterogeneities in the lower continental crust of the Erguna Massif during the Mesozoic, which we use as the basis of a structural and tectonic model for this region.     

Isotopic geochemistry (O,Sr, Pb) of the Golda Zuelva and Mboutou anorogenic complexes,North Cameroun: mantle origin with evidence for crustal contamination

The annular (6–8 km diameter) Golda Zuelva and Mboutou anorogenic complexes of North Cameroun are composed of a suite of alkaline plutonic rocks ranging from olivine gabbro to amphibole and biotite granite. For the Mboutou complex there are two overlapping centres. In the Golda Zuelva complex the plutonic rocks are associated with a later hawaiite to rhyolite volcanic suite. A Rb/Sr whole rock isochron gives an age of 66±3 Ma for the Golda Zuelva granites, with initial⁸⁷Sr/⁸⁶Sr ratio of 0.7020, and demonstrates that plutonism and volcanism were essentially contemporaneous and probably cogenetic. For Golda Zuelva and the north Mboutou centre¹⁸O/¹⁶O (5.6–6.2),⁸⁷Sr/⁸⁶Sr (0.7030–0.7045) and Pb isotopic ratios (²⁰⁷Pb/²⁰⁴Pb: 15.60–15.64) support a mantle origin for the initial magmas. Unlike Sr isotopes, the O isotopic ratios of the granitic end members at Golda Zuelva (～7.5) indicate crustal contamination. Post-magmatic alteration was not significant.For the younger south Mboutou centre the O-, Sr- and Pb-isotopic data indicate more extensive magma-crust interaction and in a different (higher level?) crustal environment with δ¹⁸O granite=3.3‰,⁸⁷Sr/⁸⁶Sr ratios up to 0.706 and Pb isotopic ratios more markedly displaced from the oceanic volcanic field. The low-¹⁸O granites probably record, at least in part, a magmatic process with subsequent minor post-magmatic alteration effects. The major and trace element systematics between the north and south Mboutou centres are directly comparable. The evolution of the magmas were dominated by fractional crystallisation and progressive crustal contamination processes.     

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.     

Aoyitake plagiogranite in western Tarim Block, NW China: Age, geochemistry, petrogenesis and its tectonic implications

SHRIMPP U-Pb zircon age and geochemical and Nd isotopic data are reported for the Aoyitake plagiogranite in western Tarim Block, NW China. The plagiogranite intruded the Middle Pro- terozoic and Lower Carboniferous with an exposure area of ca. 60 km2 and crystallized at 330.7±4.8 Ma. Rock types mainly include tonalite, trondhjemite and minor amounts of diorite and quartz-diorite. Feldspars in the rocks are dominated by oligoclase-andesine, and minor perthite observed locally. The granites are sodic with Na/K ratios (molar) between 4 and 87. Total REE (50-220 ppm) show a clear positive correlation with SiO2. There is no LRRE/HREE fractionation (LaN/YbN=0.5-1.5), me- dium negative Eu anomalies (δ Eu=0.3-0.6), high Y content and low Sr/Y ratio (~1.0). These granites exhibit relatively juvenile Nd T2DM model ages of 470 to 580 Ma and positive εNd(331 Ma) values of 6.23 to 7.65. The aforementioned characteristics are similar to those of ocean island or ocean ridge plagiogranites. However, the regional geology, especially its scale, precludes that the plagiogranite pluton was derived directly from fractionational crystallization of mantle-derived basaltic magma. We interpreted that the primary magma of the pluton might be tonalitic in composition generated by ca. 50% partial melting of the juvenile basaltic crust. The primary magma experienced intensive frac- tionational crystallization, and intruded into the middle to upper crusts to form the granite pluton. In combination with the previous regional geological data, it is concluded that the plagiogranite pluton was emplaced within the Tarim Block in respond to the Carboniferous continental rifting along the Tianshan orogenic belt.