Depletion In HFSE Research Articles

Multiple sources and magmatic evolution of the Late Triassic Daocheng batholith in the Yidun Terrane: Implications for evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau

Abstract Granitoids with diverse composition and tectonic settings provide important tools for exploring crustal evolution and regional geodynamic history. Here we present an integrated study using petrological, mineralogical, zircon U-Pb geochronological, whole-rock geochemical, and isotopic data on the Late Triassic Daocheng batholith in the Yidun Terrane with a view to understanding the petrogenesis of a compositionally diverse batholith and its implications for the evolution of the Paleo-Tethys Ocean in the eastern Tibetan Plateau. The different lithological units of the batholith, including granodiorite, monzogranite, and quartz diorite, with abundant mafic microgranular enclaves in the granodiorite (MME I) and monzogranite (MME II), show identical crystallization ages of 218–215 Ma. The mineral assemblage and chemical composition of the granodiorite are identical to those of tonalitic-granodioritic melts generated under water-unsaturated conditions. The insignificant Eu anomalies and low magmatic temperatures indicate hydrous melting in the source. The relatively narrow range of whole-rock chemical and Sr-Nd isotopes, as well as the zircon trace element and Hf isotopic compositions of the granodiorite, suggest a homogeneous crustal source for the magma. Our modeling suggests that the rock was produced by 20–50% of lower crustal melting. The Daocheng monzogranites display more evolved compositions and larger variations in Sr-Nd-Hf isotopes than the granodiorite, which are attributed to assimilation and the fractional crystallization process. This is evidenced by the presence of metasedimentary enclave and inherited zircon grains with Neoproterozoic and Paleozoic ages, a non-cotectic trend in composition, and the trend shown by the modeling of initial 87Sr/86Sr ratios and Sr. The quartz diorites and MMEs showing composition similar to that of andesitic primary magma have high zircon εHf(t) values and are characterized by enrichment in LILEs and depletion of HFSEs. They were derived from the partial melting of lithospheric mantle that had been metasomatized by slab melts and fluids. The MMEs in both rocks display typical igneous texture and higher rare earth element (REE) and incompatible element concentrations than their host granites. The presence of fine-grained margins, acicular apatite, and plagioclase megacrysts suggests a magma mingling process. The overgrowth of amphibole around the pyroxene, quartz ocelli rimmed by biotite, and oscillatory zones of plagioclase are all indicative of chemical diffusion. Their enriched Sr-Nd isotopes imply isotopic equilibrium with the host granites. Based on a comparison with the coeval subduction-related magmatism, we propose that subduction and subsequent rollback of the Paleo-Tethys (Garzê-Litang Ocean) oceanic slab was the possible mechanism that triggered the diverse Triassic magmatism within the eastern Tibetan Plateau.

High temperature fluid-rock interaction recorded in a serpentinized wehrlite from eastern Singhbhum Craton, India: Evidence from mineralogy, geochemistry and in situ trace elements of clinopyroxene

A N-S trending, lenticular, and dismembered ultramafic unit is found emplaced within the multiply deformed metapelites of the North Singhbhum Mobile Belt, belonging to the Singhbhum Craton, eastern India. Their occurrence in an orogenic belt, lack of evidences of intrusive activity and close proximity to the Singhbhum Shear Zone suggest emplacement by thrust control. These rocks have experienced extensive serpentinization which has obscured and obliterated some of the features of the protolith. Relict areas of the Bangriposi wehrlite rocks indicate that the protolith might have been a cumulate clinopyroxene-dunite/wehrlite from an oceanic magma chamber. Detailed petrography and mineralogy reveal numerous metasomatic features which are separate from the serpentinization event: (i)Very high Ni-content of the relict olivines which does not correlate with Fo contents, (ii) Presence of secondary clinopyroxene (Cpx2) rims around primary olivine and replacing primary clinopyroxene (Cpx1), which have major and trace element compositions similar to clinopyroxenes from diopsidites of crustal origin (iii) Occurrence of secondary calcic hornblendes as tiny blebs within the Cpx2 rims (iv) Occurrence of secondary Mg-chlorite as interstitial patches between olivine rimmed by Cpx2 and inter-mixed with antigorite in the matrix, (v) Modifications of original Cr-spinels to ferri-chromite cores rimmed by magnetite. These features indicate these rocks have experienced fluid-rock interaction at temperatures above the serpentinization process. Primary Cpx1 grains exhibit elevated LREE compositions and can either reflect effects of cryptic metasomatism or crystallization from an enriched melt. Both groups of clinopyroxenes exhibit LILE enrichment and HFSE depletion. P-T estimates from Cpx1 and Cpx2 yield values of ~1200 °C and 0.5–1.2 GPa. The calcic hornblendes provide more realistic estimates of ~1000 °C and ~ 1.0 GPa, which suggest that the Cpx2 + hornblende assemblage formed during high temperature fluid-rock interaction. Presence of interstitial chlorite, chlorite+antigorite in the matrix and Cr-spinel recrystallization suggests successively decreasing temperatures during the fluid-rock interaction episode. Modeled trace element characteristics of fluids which are supposed to be in equilibrium with Cpx2 show similarities with high-temperature, ore-forming basinal brines. Similar basinal brines formed Cu-U ores in the Singhbhum Shear Zone and was likely the metasomatic agent of the Bangriposi wehrlite rocks. These rocks were originally cumulate ultramafic units from an oceanic crustal magma chamber, which experienced prolonged fluid-rock interaction in a retrogressive regime, until its emplacement as a dismembered unit in an orogenic belt.

Petrogenesis and tectonic implications of two‐stage Cretaceous bimodal volcanism in the coast of Zhejiang Province, South‐east China

Shipu volcanic area is located at the east of the Jiangshan‐Shaoxing Fault zone, an important part of the Mesozoic volcanic belt in the coastal south‐east China, which provides an excellent opportunity to study the origin of rarely outcropped and coexisting bimodal volcanic rocks and bimodal dyke swarms. Two stages of bimodal volcanic rocks and dyke swarms yield zircon U–Pb ages of ca. 118–121 Ma and ca. 95–93 Ma, respectively. Both bimodal magmatic rocks show enrichment in LILEs and LREEs, but depletion in HFSEs. Basalts of the early‐stage bimodal volcanism have whole‐rock (87Sr/86Sr)i ratio of 0.71121, εNd(t) value of −8.0, and zircon εHf(t) values of −19.3 to −11.7, while the contemporaneous rhyolites show whole‐rock (87Sr/86Sr)i ratios of 0.71023 to 0.71106, εNd(t) values of −7.5 to −6.6 and zircon εHf(t) values of −15.3 to −7.6 with TDMC of 0.96–2.11 Ga. Basaltic dykes of the late‐stage bimodal volcanism have whole‐rock (87Sr/86Sr)i ratios of 0.70898 to 0.70908, εNd(t) values of −5.8 to −5.7 and zircon εHf(t) values of −10.5 to 2.9, while coexisting rhyolitic dykes display whole‐rock (87Sr/86Sr)i ratios of 0.69618 to 0.71172, εNd(t) values of −3.1 to −3.0 and zircon εHf(t) values of −9.2 to 11.9 with TDMC of 0.37–1.71 Ga. These features suggest that the early‐stage basalts could be derived from partial melting of an enriched lithospheric mantle metasomatized by slab‐derived fluids, and its corresponding felsic end‐member is the product of partial melting of Proterozoic middle‐lower crust due to underplating of basaltic magmas with minor addition of mantle materials. As for the late‐stage bimodal volcanism, the basaltic dykes are also derived from an enriched mantle similar to the early‐stage counterpart. However, the rhyolitic dykes mainly originated from partial melting of ancient lower crustal materials with abundant input from the depleted mantle. As a whole, two stages of bimodal magmatism in Shipu area provide direct constraints on regional tectonic transition from an extensional arc setting to a back‐arc setting.

Field, textural, geochemical, and isotopic constraints on the origin and evolution of the magmatic microgranular enclaves from the Gharib Granitoid Complex, North Eastern Desert, Egypt

The Neoproterozoic Gharib Granitoid Complex (GGC), in the North Eastern Desert (NED) of Egypt, is characterized by the occurrence of varieties of granitoids. Monzogranite, the most abundant granite variety, contains abundant magmatic microgranular enclaves (MMEs) of mafic to intermediate composition. The MMEs are more mafic than their host and marked by a greater amount of amphibole and biotite. They are finer grained and have porphyritic/poikilitic textures.The sharp, crenulated, and fine-grained quenched contacts of the elongated, rounded, and ellipsoid form of the MMEs with their felsic host indicate rapid cooling during a magma mixing/mingling event. Micro-textural evidence supports the mechanical transfer of minerals that crystallized in felsic magma to the mafic melt (e.g., feldspar phenocrysts cross-cutting the MME-host boundaries, stubby apatite, quartz ocelli, and poikilitic outermost rims of K-feldspar phenocrysts), supercooling/quenching process (e.g., acicular apatite and elongated hornblende and biotite crystals), and disequilibrium growth (e.g., hornblende-biotite clots replaced early crystallized clinopyroxene), which also favour a magma mingling/mixing origin. Linear to curvilinear chemical variations of the MMEs and the host monzogranites on Harker diagrams, highly similar trace element contents, and Sr-Nd isotopic systematics together with mineral-chemical features also support mixing/mingling as the main process in magma genesis and strongly suggest that the MMEs were supercooled hybrid globules within cooler, partially crystallized host felsic magma. The crust-like geochemical signatures (i.e., SiO2 content, metaluminous nature, and lack of upper crustal xenoliths and typical peraluminous minerals), along with low [MMEs: (87Sr/86Sr)initial = 0.70136–0.70373; monzogranites: (87Sr/86Sr)initial = 0.70202–0.70320)] along with zircon and apatite U-Pb age data indicate the possible involvement of juvenile lower continental crust (LCC) of the Arabian-Nubian Shield (ANS) in their genesis. The medium- to high-K calc-alkaline affinity, the LILE enrichment, and the HFSE depletion along with low Nb/La ratios and positive εNd(t) (MMEs = +4.01 - +5.62, monzogranites = +2.29 - +6.03) indicate the contribution of depleted lithospheric mantle in the magma genesis, whereas the low δEu and fairly high δCe values, stable La/Sm, and variable Sr/Th ratios in apatites from the monzogranites and MMEs indicate the contribution of volatile/fluids from the remnants of the oceanic slab in facilitating melting in the source region. The genesis of the monzogranites and MMEs started with asthenosphere upwelling, followed by melting of the remnants of lithospheric mantle, and underplating of mantle magma, which led to partial melting of the LCC. Multi-element modelling postulates that the low degree mixing between partially crystallized mantle magma and LCC-derived felsic melt (Fmix ∼ 0.1) could produce the parent magma for the monzogranites. Further mixing/mingling of the crystal-charged monzogranite parent magma with new pulses of mafic melts (Fmix ∼ 0.25) developed a hybrid zone and formed the hybrid MMEs. Thus, the final episode of the ANS evolution was associated with a complex post-collisional interplay between asthenosphere, lithospheric mantle, and the LCC caused by the removal of delaminated lithospheric root and dense lower crust following the collision between East and West Gondwana in the Neoproterozoic time.

Late Paleozoic multi-stage subduction accretion of the southwestern Central Asian Orogenic Belt: insights from the Late Carboniferous-Early Permian granites in the southern West Junggar, NW China

ABSTRACT The Late Carboniferous to Early Permian tectonic evolution of the South West Junggar (SWJ), northwest China, a crucial constituent in the southwestern domain of the Central Asian Orogenic Belt (CAOB), has long been the subject of debate. Here, detailed petrographic studies, new nine LA-ICP-MS zircon U-Pb ages and forty-nine whole-rock geochemical results from six granitic plutons in the SWJ were combined with previously available data to elucidate their petrogenesis and re-decipher Late Palaeozoic tectonic evolution and accretionary process of the southwestern CAOB. The studied granites can be geochemically classified as I- and A-type granites, and the former is further subdivided into I-type and highly fractionated I-type granites. The calc-alkaline I-type amphibole-bearing granites (Group I) yield crystallization ages of ca. 317–315 Ma, and exhibit depletions in HFSEs and enrichments in LILEs with no to slight Eu anomalies and low zircon saturation temperatures, indicating that the formation of these granites may be closely related to the subduction of the Junggar oceanic slab. The Group II granite porphyries (ca. 312 Ma) contain negligible amphibole, and are characterized by evolved SiO2 but low zircon saturation temperatures with sharply negative Eu, Ba, Nb, Ta, Sr, P and Ti anomalies, belonging to highly fractionated I-type granites, and were probably produced in a tectonic transition from compressional to extensional setting. In contrast, the latest Carboniferous (ca. 304–298 Ma) alkaline A-type granites (Group III) are amphibole-absent, and are similar to Group II granites in major element compositions, but exhibit higher zircon saturation temperatures, indicating that they probably resulted in a back-arc extensional setting triggered by roll-back of the subducting Junggar oceanic slab since ca. 310 Ma. In combination with regional geology and re-interpretation of previous data, a multi-stage subduction-accretion model is suggested to account for the Late Palaeozoic tectonic evolution of the southwestern CAOB. This model can plausibly interpret the differences in duration of subduction between the northern and southern domains of West Junggar and the coexistence of A-type granites with subduction-related sanukites in the SWJ during Carboniferous to Permian.

Petrogenetic links between rare metal-bearing pegmatites and TTG gneisses in the West African Craton: The Mangodara district of SW Burkina Faso

We describe the geological context of rare metal-bearing pegmatites from the Mangodara district (South-West Burkina Faso, Paleoproterozoic West African Craton) and discuss their petrogenesis and links with the host rocks. The Mangodara district exposes a gneiss-granitoid complex structured in a regional-scale dome, mantled by granodioritic gneiss enclosing rafts of amphibolite, micaschist and paragneiss, and cored by tonalitic to trondhjemitic gneisses. These gneisses enclose granitoid plutons, and four populations of rare metal-bearing pegmatites: titanite-allanite-, apatite-zircon-, garnet-columbite (Li, Nb) and garnet-REE (Ti, Y, HREE)-bearing varieties.Rafts of migmatitic amphibolite, micaschist and paragneiss are chemically equivalent to nearby Birimian greenstone belts. An origin of the gneiss-granitoid complex by partial melting is suggested by diffuse contacts of rafts with the gneisses as well as the presence of garnet-bearing or hornblende-bearing leucosome in paragneiss and in amphibolites, respectively. Textural continuity between pegmatites and granitic veins concordant to the foliation of the gneisses point to syntectonic segregation of the pegmatite-forming magmas.The compositions of gneisses and plutonic rocks spread between a Na-rich pole and a K-rich pole. Granodioritic gneiss, hornblende-biotite granodiorite and potassic biotite granitoids define a K-rich series attributed to relatively low-pressure high-degree partial melting of dominantly paragneiss, which accounts for low to absent LILE and HFSE fractionation compared to the paragneiss. Tonalitic-trondhjemitic gneiss, a biotite trondhjemite and a peraluminous two-mica trondhjemite belong to a Na-rich series characterized by low K content and strong depletion in REE and HFSE, which might reflect plagioclase clustering after partial melting at a relatively high pressure of amphibolite, and fractionation of HFSE-REE-bearing minerals. Thermodynamic and mass balance modeling further corroborate these propositions implying that partial melting of paragneiss and amphibolite at about 5 to 10 kbars for a temperature of 780 to 850 °C followed by melt/solid segregation and fractional crystallization are susceptible to generate the Na- and K-rich series.U-Pb dating of zircon from a titanite-allanite-bearing pegmatite at 2094.3 ± 8.8 Ma and of apatite from tonalitic-trondhjemitic gneiss, apatite-zircon-bearing pegmatite and granodioritic gneiss at 2094 ± 21 Ma, 2055 ± 20 Ma, and 2041 ± 33 Ma respectively, confirm that partial melting, melt segregation and crystallization-cooling occurred during the Eburnean orogeny.Based on these data, we propose that (i) titanite-allanite- and apatite-zircon-bearing pegmatites result from syntectonic segregation of residual melt during crystallization of the tonalitic-trondhjemitic gneiss, (ii) garnet-columbite-bearing pegmatites originate from syntectonic melt segregation from the migmatitic paragneiss, and (iii) garnet-REE-bearing pegmatites derive from segregation of a residual melt within the granodioritic gneiss.

Early Jurassic S-type granitoids in the Nyainqêntanglha Range, South Tibet: A record of slab roll-back of subducted Neo-Tethyan Ocean

The Gangdese orogenic belt in southern Tibet records a succession of arc-trench-basin assemblages formed by the Neo-Tethyan subduction and the subsequent India-Asian collision. However, the geochronology and geodynamics of S-type granites in the northern part of the Gangdese belt are disputed. This paper describes ~190 Ma S-type granitoids outcropped in the Nyainqêntanglha Range, South Tibet, focusing on their petrogenesis and their relationship with contemporaneous magmatism. There are three distinct groups of granitoids, based on field relationships, petrology, and geochemistry. The coarse-grained Group I and II granitoids are large, sheet-like bodies and exhibit high contents of incompatible elements and negative Eu anomalies. The fine-grained Group III granites form small laccoliths and have low REE contents and positive Eu anomalies. The negative εNd(t) and εHf(t) values, abundant inherited zircons, moderately light REE and LILE enrichment, and HFSE depletion indicate that the Damxung granitoids were derived mainly by the partial melting of a metapelitic source. Further studies proposed that group I and II samples with high contents of incompatible elements and negative Eu anomalies represent fractionated melts, whereas group III samples with low REE contents and positive Eu anomalies represent cumulates. The age distribution pattern of inherited zircons supports the interpretation that the Lhasa terrane rifted away from the northern margin of the Indian plate. Considering previously published data in the Lhasa terrane, we suggest that the Early Jurassic magmatism could be related to a period of subducted oceanic slab roll-back, which provides new insights into the evolution process of the Neo-Tethyan ocean realm.

Petrogenesis of Early Carboniferous Alkaline Basalt from the Wusun Mountain: Implications for Tectonic Evolution of the Western Yining Block, NW China

AbstractLate Paleozoic volcanic rocks are well exposed in the Yining Block, NW China, and are predominately composed of andesites, rhyolites and volcaniclastics as well as minor basalts. Study of the petrology, whole‐rock geochemistry and zircon U‐Pb dating for the Early Carboniferous alkaline basalts from Wusun Mountain, western Yining Block, constrains their petrogenesis and tectonic evolution. The alkaline basalts consist mainly of plagioclases, mostly albite and labradorite, as well as clinopyroxenes and olivines; zircon U‐Pb dating indicates their formation at ca. 350 Ma. Geochemically, the basaltic samples have low SiO2 contents, and high TiO2, Al2O3 and alkaline contents, coupled with high Na2O/K2O ratios, displaying an alkaline basalt affinity. They show remarkable LILE enrichment and HFSE depletion. Meantime, these samples have relatively high TFe2O3, MgO, and Mg# values as well as Ni and Cr, relatively high Sm/Yb and U/Th, suggesting origination from a mantle source metasomatized by slab fluids. They formed in a transitional tectonic setting from arc to intraplate, showing a typical affinity of back‐arc basin basalts. The alkaline basalts were likely generated in a nascent back‐arc extension setting resulting from slab rollback of the southern Tianshan oceanic lithosphere. A bi‐directional subduction model seems more reasonable for the evolution of the southern Tianshan Ocean. These new data will provide a new tectonic model for Late Paleozoic tectonic evolution of the western Yining Block.

Melt-rock interaction in the lower crust based on silicate melt inclusions in mafic garnet granulite xenoliths, Bakony–Balaton Highland

Major and trace element composition of silicate melt inclusions (SMI) and their rock-forming minerals were studied in mafic garnet granulite xenoliths from the Bakony–Balaton Highland Volcanic Field (Western-Hungary). Primary SMIs occur in clinopyroxene and plagioclase in the plagioclase-rich domains of mafic garnet granulites and in ilmenite in the vicinity of these domains in the wall rock. Based on major and trace elements, we demonstrated that the SMIs have no connection with the xenolith-hosting alkaline basalt as they have rhyodacitic composition with a distinct REE pattern, negative Sr anomaly, and HFSE depletion. The trace element characteristics suggest that the clinopyroxene hosted SMIs are the closest representation of the original melt percolated in the lower crust. In contrast, the plagioclase and ilmenite hosted SMIs are products of interaction between the silicic melt and the wall rock garnet granulite. A further product of this interaction is the clinopyroxene–ilmenite±plagioclase symplectite. Textural observations and mass ­balance calculations reveal that the reaction between titanite and the silicate melt led to the formation of these assemblages. We propose that a tectonic mélange of metapelites and (MOR-related) metabasalts partially melted at 0.3–0.5 GPa to form a dacitic–rhyodacitic melt leaving behind a garnet-free, plagioclase+clinopyroxene+orthopyroxene+ilmenite residuum. The composition of the SMIs (both major and trace elements) is similar to those from the middle Miocene calc-alkaline magmas, widely known from the northern Pannonian Basin (Börzsöny and Visegrád Mts., Cserhát and Mátra volcanic areas and Central Slovakian VF), but the SMIs are probably the result of a later, local process. The study of these SMIs also highlights how crustal contamination changes magma compositions during asthenospheric Miocene ascent.

Petrogenetic evolution of the Zhuopan potassic alkaline complex, western Yunnan, SW China: Implications for heterogeneous metasomatism of lithospheric mantle beneath Simao and western Yangtze block

Potassic alkaline rocks are instrumental in unveiling the composition and geodynamic evolution of mantle lithosphere. The Zhuopan alkaline complex, located in the western Simao block, SW China, exhibits features of high potassium, enrichment in LILEs, and depletion in HFSEs. These same elemental features and SrNd isotopic compositions are also evident in the coeval clinopyroxenitic and syenitic rocks, indicating a comagmatic origin. In this paper, we present new data and discuss the petrogenesis of these rocks. We show that magmas parental to the Zhuopan alkaline complex are derived from highly enriched clinopyroxene-rich lithospheric mantle. These alkaline magmas emplaced in shallow crustal level and experienced crustal assimilation during ascent and through magma chamber processes. Pyroxenite is of cumulitic origin dominated by clinopyroxene and apatite. It formed at an early stage and received more effective crustal contamination. Pyroxene syenite, syenite and nepheline syenite are compositional mixture of variably evolved melt with incompletely segregated liquidus phases (e.g., clinopyroxene, biotite, alkali-feldspar, apatite, nepheline). An important observation is that the Dy/Yb ratio, LILE and LREE abundances of the Zhuopan complex are significantly higher than those of the contemporaneous mafic volcanic rocks in the western Yangtze block, reflecting different sources and source histories. The lithospheric mantle beneath Simao and western Yangtze blocks must have been heterogeneously metasomatized by oceanic subduction-related fluids and melts. Particularly, the mantle source for the Zhuopan alkaline rocks exhibits feature of carbonatite metasomatism. This is further supported by our modeling where melts derived from marine carbonate play a crucial role.

Geochronology and Sr-Nd-Pb-Hf-O isotope geochemistry of Miocene intrusive rocks from Tsushima Islands, Japan: Constraints on petrogenesis and tectonic setting

We present geochemical and Sr-Nd-Pb-Hf-O isotopic data and zircon U-Pb ages for Miocene intrusive rocks (granitoids, mafic microgranular enclaves (MMEs), and diabases) from Tsushima Islands between Kyushu (Japan) and southeastern Korea. A granite yielded an age of 16.23 ± 0.16 Ma. The granitoids are mostly I-type, but are transitional between metaluminous and peraluminous compositions. All samples exhibit enrichment in LREE and LILE (e.g., K, Rb, and Pb), and depletion in HFSE (e.g., Nb, Ta, and Ti), implying formation in a subduction-related tectonic setting. However, on tectonic discrimination diagrams, data for all samples plot between the orogenic and anorogenic fields, indicating that Tsushima Islands were in a transitional tectonic regime from subduction to continental extension at ca. 16 Ma. The δ18Oclinopyroxene values of a diabase range from 5.65‰ to 5.84‰. The Sr-Nd-Pb isotopic compositions of the diabases (87Sr/86Sr = 0.70423–0.70481; εNd = −0.8 to +3.6; 206Pb/204Pb = 18.27–18.40) are within the range of Miocene Eoil basalts from the Cenozoic Yangnam Basin in southeastern Korea, indicating the mantle source was a metasomatized mantle wedge. The granitoids are characterized by more enriched isotopic compositions than the diabases (87Sr/86Sr = 0.70611–0.70756; εNd = −6.0 to −3.8; 206Pb/204Pb = 18.43–18.55), and the MMEs have an intermediate isotopic composition. δ18Oquartz values of the granitoids vary from 8.68‰ to 9.25‰, and zircons from a granite have εHf(t) values of −1.7 ± 1.2. The MMEs may have formed by partial melting of amphibolitic lower crust, whereas the granitoids possibly had a hybrid source consisting of amphibolite and graywacke.

Early Cretaceous basalts record the modification of the North China Craton lithospheric mantle: implications for lithospheric thinning

ABSTRACT The modification of the Archaean lithospheric mantle root beneath the eastern North China Craton (NCC) has been noticed. However, the degree of modification and the characteristics of metasomatic agents for the NCC lithospheric mantle are still unclear. Here, we compile the geochemistry and 87Sr/86Sr and 143Nd/144Nd isotope ratios of Cretaceous primitive basalts (Yixian, Sihetun, Fangcheng and Feixian) from the eastern NCC and estimate the inputs of subduction-related elements into the Archaean lithospheric mantle beneath the NCC. These basalts show primitive (high MgO, Cr and Ni contents) and arc-like geochemical features (enrichments in LILEs and depletions in HFSEs) that indicate that the mantle sources were modified by fluids related to subducted crustal materials. Substantial proportions of subduction-mobile elements (e.g. ~95% Ba and ~90% Th) transferred to the basalt sources via hydrous melts. Thus, a large volume of fluids transferred into the lithospheric mantle. The data support the model that the NCC Archaean lithospheric mantle was weakened by hydrous melts, which resulted in a fusible weakened lithospheric mantle. Preliminary lithospheric thinning was induced by the extension of the NCC resulting from trench retreat of the Paleo-Pacific plate at ~140-120 Ma. Then, decompression melting of the lithospheric mantle caused pervasive melts in the weakened lithospheric mantle, resulting in the lithospheric mantle having a low viscosity comparable to the asthenospheric mantle, which principally reduced the lithospheric mantle.

A non-arc tectonic setting for the evolution of Archean gabbro anorthosite Complexes: Evidence from the Singhbhum Craton, eastern India

Understanding the petrogenetic evolution of Archean gabbroic cumulates provides a window to the mantle geodynamics operated in the early Earth. Gabbroic cumulates are associated with various tectonic settings including convergent plate boundaries, mantle plumes, mid-ocean ridges, anorogenic, post-orogenic, continental rift settings and oceanic plateaus. The exposures of preserved gabbroic suites in the Archean are limited. The Mesoarchean Mayurbhanj Gabbro Anorthosite Complex (GAC) is a well-preserved mafic body situated along the eastern margin of the craton along with concomitant anorogenic Mayurbhanj granite. These rocks are devoid of deformational structures and/or mineral recrystallization. Based on their mineralogical characteristics, GAC rocks are mainly grouped into gabbronorite, leucogabbronorite, leucogabbro and gabbro. The minimal abundance of magmatic amphibole and the variation in the anorthite content (An36-An84) rules out the role of a hydrous mantle source in the evolution of these rocks. The studied samples display tholeiitic trend transitioning to calc-alkaline field, major and trace element data indicates the fractional crystallization of olivine and pyroxene. The trace element systematics of the Mayurbhanj GAC show LILE enrichment over HFSE depletion, with negative Nb-Ta and mixed Zr-Hf and Ti anomalies. The geochemical signatures such as elevated Th/Yb, Nb/Yb, variable Dy/Yb and Dy/Dy* and varying Gd/YbCN ratios suggest that parental magmas of the Mayurbhanj GAC originated from a heterogeneous depleted mantle source. The studied rocks' Sr-Nd isotopic systematics reveal that their parental magma has undergone assimilation by a Paleoarchean felsic crust. We argue that the ‘arc-like’ geochemical signatures displayed by these rocks resulted from crustal contamination and may not be due to subduction origin. Consolidating the field, geochemical and isotopic evidence suggests that the Mesoarchean Mayurbhanj GAC possibly evolved from an anhydrous heterogeneous mantle source in a non-compressive tectonic regime.

Geochemistry and petrogenesis of mafic volcanic rocks from Kahak area (Urumieh-Dokhtar magmatic arc)

The Kahak mafic volcanic rocks in the central part of the Urumieh-Dokhtar Magmatic Arc are composed of ‎basalts‏ ‏and‏ ‏basaltic ‎andesite and show sub alkaline to transitional affinity. They are calc-alkaline based on the tholeiitic index (THL). U-‎Pb zircon dating ‎yields almost 60‎‏ ‏‎(Middle Paleocene) and 24 to 19 Ma (Late Oligocene–Early Miocene) for andesitic basalt and basaltic ‎rocks ‎respectively. These rocks ‎are identified by LREE and LILE enrichment and HFSE depletion with relatively negative or without ‎Eu ‎anomalies and E-MORB like ‎pattern in multiple spider diagrams that.attributed to the subduction of the Neotethyan oceanic ‎slab ‎beneath the central Iranian microcontinent. Based on petrography, trace and rare earth elements, and isotopic features, ‎fractional ‎crystallization played a significant role ‎during magma evolution in these rocks. Trace element modeling suggests that the ‎studied mafic ‎rocks were derived by partial melting ‎within the spinel‏ ‏lherzolite mantle. Isotopic ratios also show that they resulted from ‎lithospheric ‎mantle metasomatized by ‎released fluids from subducted slab sediments. The studied samples might have formed in the ‎extensional regime ‎followed ‎by slab rollback and undergone a continental arc to back-arc basin transition during the Paleocene to ‎Miocene. This basin might ‎have ‎been closed in the‏ ‏middle Miocene.

Petrological and geochemical characteristics of the diabase and metasomatised dikes from the Tekirova ophiolite (SW Anatolia, Turkey): Tectonomagmatic evolution of the southern Neotethys

Ophiolites are widespread across the Anatolian plate and formed during the termination of different branches of the Neotethys ocean. The Tekirova ophiolite, situated in the Antalya ophiolite nappes in Southwest Anatolia, originated in the southern branch of the Neotethys. The former ophiolite consists of mantle and crustal sequences, in which, it is predominantly crosscut by the isolated diabasic dikes. Here, we have distinguished two dike types on the basis of their petrographic features and also degree of alteration. The relatively fresh types are generally named as diabase dikes. While the altered types are called as metasomatised dikes, and these dikes display ophitic to subophitic textures. The diabase and metasomatised dikes show enrichment in LILEs and depletion in HFSEs, indicating characteristics of suprasubduction zone ophiolites. This depletion of mobile elements may have been caused by metasomatism. The HFSEs versus HREEs diagrams show that the dikes were formed from 10 to 30% melting of extracted melts from previous mantle melting in which modified by subduction-derived fluids.

Geochronology, geochemistry, and isotope compositions of “Grenvillian” S-type granites in the North Qinling unit, central China: Petrogenesis and tectonic significance

An integrated study of whole-rock elements, Sr–Nd isotopes, and zircon U–Pb ages and Hf-O isotopes was carried out for the “Grenvillian” granites from the North Qinling unit, central China. The results not only place clear constraints on the petrogenesis of these granites, but also give insights into the geodynamic evolution of the Qinling orogen during the assemblage of supercontinent Rodinia. LA-ICPMS zircon U–Pb dating of three representative granites from the Dehe, Fangzhuang, and Zhaigen outcrops yielded formation ages of ca. 940–900 Ma. These muscovite-bearing granites have high silica contents (72–79 wt%) and are weak to strong peraluminous (ASI = 1.01–1.15). They exhibit arc-like trace element distribution patterns with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion in HFSE (e.g., Nb, Ta, Ti). Besides, they show variable initial 87Sr/86Sr ratios of 0.704 to 0.729 and relatively low εNd(t) values of −3.2 to −6.1, as well as variable zircon εHf(t) and δ18O values (−8.1–+2.6, 7.9‰–12.0‰). These results suggest their derivation from partial melting of a heterogeneous source that dominated by supracrustal components. Furthermore, a few inherited grains give negative εHf(t) values of −9.4 to −18.4 with two-stage Hf model ages up to 2.9 Ga, suggesting the involvement of minor Archean crustal components. Along with their peraluminous features, high zircon δ18O values, and enriched Sr–Nd isotope compositions, these granites were unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. Geothermometric and compositional approaches suggest these S-type granites were likely generated by dehydration melting of metagraywacke- and/or metapelite-dominated sources. They might be formed in an accretionary orogen associated with the Rodinia-assembly during the early Neoproterozoic, through which the outboard North Qinling terrane was eventually adjacent to the South China Block.

Early Neoproterozoic continuous oceanic subduction along the northern margin of the Tarim Block: Insights from ca. 910–870 Ma arc-related magmatism in the Aksu area, NW China

The Tarim Block in NW China records the tectonic transition from compression to extension during the Neoproterozoic, and hence is regarded as a component of the Rodinia supercontinent. To unravel the role of the Tarim Block in the Rodinia assembly during the early Neoproterozoic, new geochronological and geochemical analyses were conducted on the gabbroic diorite, plagiogranite and andesite exposed in the Aksu area, northwestern Tarim. Zircon U-Pb dating indicates that these magmatic rocks were formed at ca. 900–870 Ma. Geochemically, these magmatic assemblages have typical features of arc-type rocks, showing enrichment of LILEs and depletion of HFSEs (such as Nb, Ta and Ti). They all have positive zircon εHf(t) values (+6.8 to +12.7), positive εNd(t) values (+3.6 to +6.0), and low and constant (87Sr/86Sr)i ratios (0.70252 to 0.70485). The gabbroic diorites display geochemical features of boninitic rocks such as high SiO2 (53.4–56.3 wt%), MgO (5.79–9.15 wt%), low TiO2 (0.29–0.41 wt%) contents, indicating that they were derived by partial melting of residual mantle. The plagiogranites are characterized by high SiO2 (74.7–79.6 wt%), Na2O (3.87–4.74 wt%), moderate Al2O3 (11.5–13.2 wt%), low K2O (0.15–0.22 wt%) contents, with very low TiO2 (0.08–0.17 wt%) and total REEs (6.3–19.0 ppm) contents similar to oceanic plagiogranite, indicating a hydrous low-pressure melting of mafic protolith. The positive εNd(t) and zircon εHf(t) values and low (87Sr/86Sr)i ratios of the andesitic rocks in the Aksu area suggest a depleted mantle origin. The analysis of trace elements (e.g. Th, Zr, Nb) of these intrusive and volcanic rocks in the Aksu area indicates that their magma sources had been metasomatized by slab-derived fluids. Combined with trace element features, we infer that these early Neoproterozoic magmatic rocks in the Aksu area developed in an oceanic subduction setting with arc-back-arc system probably analogous to the Izu-Bonin (IB) subduction system in the western Pacific. These findings lead us to propose that continuous oceanic subduction beneath the northern margin of the Tarim Block occurred during ca. 910–870 Ma, corresponding to the Neoproterozoic circum-Rodinia subduction-accretion system, which provides new evidence for the accretionary process of the Tarim Block to Rodinia during the early Neoproterozoic.

Petrogenesis and tectonic setting of the Early Permian gabbro–granite complex in the southeastern Central Asian Orogenic Belt, central Inner Mongolia, China

Gabbro–granite complexes formed in active continental margins provide critical constraints on magmatic processes, crust–mantle interactions, and tectonic evolution. The petrogenesis and tectonic setting of the Early Permian igneous rocks in the southeastern Central Asian Orogenic Belt have been a subject of dispute for a long time. This contribution presents an integrated study on the field occurrence, petrology, petrography, zircon U–Pb ages, in situ zircon Hf isotopes, whole-rock geochemistry, and Sr–Nd isotopes of the Early Permian Yiheshaorong gabbro–granite complex, Xi Ujimqin Banner, central Inner Mongolia. Lithologically, the Yiheshaorong complex consists predominantly of gabbro, pyroxene diorite, and alkali feldspar granite. Results of LA–ICP–MS zircon U–Pb dating suggest that crystallization ages of the gabbro, pyroxene diorite, and alkali feldspar granite are 281–280, 278, and 281–280 Ma, respectively. Silica (SiO2) content varies from 41.26 to 79.64 wt% and exhibits a gap between 56.84 and 75.45 wt%, displaying a good bimodal geochemical distribution. Geochemically, the gabbros and pyroxene diorites are characterized by enrichment of large ion lithophile elements (LILEs; e.g., Ba and K) and depletion of high field strength elements (HFSEs; e.g., Nb, Ti, and P). These rocks show similar homogeneous zircon Hf isotopic (positive εHf (t) values from +7.8 to +10.1) and whole-rock Sr–Nd isotopic (low initial 87Sr/86Sr ratios from 0.7044 to 0.7051, positive εNd (t) values from +3.7 to +4.7) compositions. The gabbros and pyroxene diorites originated from partial melting of depleted asthenospheric mantle metasomatized by subduction slab fluids; and they experienced various degrees of crystallization differentiation during ascension. The alkali feldspar granites are ferroan, alkali-calcic–calc-alkalic series rocks marked by high SiO2, K2O + Na2O, and FeOT/MgO; and low Al2O3, CaO, and MgO contents. Their trace elements are marked by enrichment of LILEs (such as K and Rb); depletion of HFSEs (such as Nb and Ti), Ba, Sr, and P; and strongly negative Eu anomalies (Eu/Eu* = 0.17–0.57), with coincident distribution patterns forming a seagull-type distribution form, showing an A-type granite affinity. The alkali feldspar granites are characterized by positive εHf (t) values (+8.6 to +10.3, young TDM2 ages, 645–758 Ma) and εNd (t) values (+3.5 to +3.8 as well as young TDM2 ages, 667–687 Ma). All these geochemical characteristics indicate that the alkali feldspar granites originated from the partial melting of juvenile crust under high temperature and low pressure condition. The Yiheshaorong complex was most likely formed in a back-arc spreading basin. Such an extensional regime can likely be attributed to the upwelling of asthenospheric mantle caused by the rollback and break-off of a subducted Paleo-Asian Ocean slab in the Early Permian, which induced large-scale extension and magmatism in the study area.

Zircon U–Pb geochronology, Hf isotope composition, and petrochemical characteristics of Paleocene granitoids in the western Gangdese Belt, Tibet

Abstract —The research team studied the petrology, whole-rock geochemistry, zircon U–Pb age, and stable isotopic characteristics of the Rongguo Longba and Garongcuo granites of the Nuocang area to understand better the impact of Neo-Tethys ocean subduction and In-dia–Eurasia continental collision on Paleocene tectonomagmatic processes along the southern margin of the Gangdese Belt. The Rongguo Longba granite and Garongcuo granite porphyry formed at 61.86 and 62.17 Ma, respectively. The Nuocang granitoids are characterized by (1) high SiO2, NaO2, and Al2O3 contents and low FeOtot, MgO, and TiO2 contents; (2) LREE and LILE enrichment and HREE and HFSE (Nb, P, and Ti) depletion; and (3) obvious negative Eu anomalies. These features indicate that the Nuocang granites are of the high-K calc-alkaline and peraluminous granite types. Furthermore, their zircon Hf isotope characteristics suggest that the magma source region has an ancient crystalline basement. The basaltic andesitic crystal tuff is the product of garnet–peridotite partial melting and crust contamination from rising magma emplacement.

Eocene dike swarm and felsic stock in Central Iran: Roles of metasomatized mantle wedge and Neo-Tethyan slab

The Eocene felsic stock and dike swarm with intermediate composition from Central Iran (southwest of Jandaq) are studied to understand geodynamic evolution of the Central-East Iranian Microcontinent (CEIM). Field relationships show that the dikes, composed of trachyandesite and basaltic trachyandesite, cross-cut the felsic stock. Whole rock geochemical data reveal the calc-alkaline nature of stock and high-K calc-alkaline to shoshonitic nature of dikes. The felsic stock and intermediate dikes show LREE and LILE enrichment and HFSE (Ta, Nb and Ti) depletion, suggesting that they probably formed in a subduction zone and are similar to the volcanic arc rocks. The rock samples of felsic stock have geochemical characteristics of the high silica adakites (HSA) produced by melting of a subducted oceanic crust. On the other hand, the dikes have characteristics of low silica adakites (LSA) produced by partial melting of a lithospheric mantle peridotite previously metasomatized by slab-derived melts. The Eocene felsic stock and dikes probably formed by subduction of the CEIM-confining oceanic crusts beneath the CEIM. The field and geochemical studies indicate that the magma originated from melting of basic section (amphibolite) of the subducted slab (HSA) will arrive to the Earth surface before the magma formed by melting of mantle wedge peridotites (LSA).