Mantle-normalized Trace Element Diagrams Research Articles

The Subduction-Related Metavolcanic Rocks of Maroua, Northern Cameroon: New Insights into a Neoproterozoic Continental Arc Along the Northern Margin of the Central African Fold Belt

The metavolcanic rocks around Maroua in the Far North Region of Cameroon are located at the northern margin of the Central African Fold Belt (CAFB) and have not been studied to date. The petrographic and whole-rock geochemical data presented in this paper highlight their magma genesis and geodynamic evolution. The lavas are characterized by basaltic, andesitic, and dacitic compositions and belong to the calc-alkaline medium-K and low-K tholeiite series. The mafic samples are essentially magnesian, while the felsic samples are ferroan. On a chondrite-normalized REE diagram, mafic and felsic rocks display fractionated patterns, with light REE enrichment and heavy REE depletion (LaN/YbN = 1.41–5.38). The felsic samples display a negative Eu anomaly (Eu/Eu* = 0.59–0.87), while the mafic lavas are characterized by a positive Eu anomaly (Eu/Eu* = 1.03–1.35) or an absence thereof. On a primitive mantle-normalized trace element diagram, the majority of the samples exhibit negative Ti and Nb–Ta anomalies (0.08–0.9 and 0.54–0.74, respectively). These characteristic features exhibited by the metavolcanic rocks of Maroua are similar to those of subduction-zone melts. This subduction would have taken place after the convergence between the Congo craton (Adamawa-Yadé domain) and the Saharan craton (Western Cameroonian domain). Petrological modelling using major and trace elements suggests a derivation of the Maroua volcanics from primitive parental melts generated by the 5–10% partial melting of a source containing garnet peridotite, probably generated during the interaction between the subducted continental crust and the lithospheric mantle and evolved chemically through fractional crystallization and assimilation.

Origin and timing of spilitic alterations in volcanic rocks from Głuszyca Górna in the Intra-Sudetic Basin, Poland

The formation of spilitic assemblages (i.e. chlorite and albite) has been ubiquitously involved during the evolution of continental early-Permian volcanics from the Intra-Sudetic Basin (ISB). Based on the investigation of laccolith-type and variably-altered trachyandesite exposure in the vicinity of Głuszyca Górna (Lower Silesia, Poland), we have demonstrated that apatite fission-track dating (AFT), coupled with chlorite geothermometry, can be successfully applied to denote the timing of low-temperature alterations within volcanic rocks. The primary magmatic assemblages of the trachyandesites (i.e. augite and andesine-labradorite) have been affected by chloritization and alblitization respectively, followed by the formation of secondary titanite, celadonite, and calcite. The chlorite species have crystallized in the range of 106–170 °C, that exceeds Apatite Partial Annealing Zone (70–110 °C). The secondary, nearly pure albite (Ab ~ 99 mol.%) with weak to dark-brown cathodoluminescence replaces primary plagioclase (~ An37–50Ab47–58Or2–4) along the cleavage and/or twinning planes during Al3+–conservative reaction. The accessory apatite is marked by swallow-tail terminations indicative of rapid cooling formation conditions. It shows homogenous chemical composition, high F− content, and pink to yellow (REE3+ and Mn2+-activated, respectively) cathodoluminescence. Based on the AFT dating, the development of spilitic alterations within the early-Permian (ca 290 Ma) laccolith from Głuszyca could not only span the range of 182–161 Ma (Middle Jurassic), but also occurred prior to large-scale geological events in the ISB, such as burial under late-Mesozoic sediments, as well as tectonic inversion and exhumation. Whole-rock geochemistry of trachyandesites altered to various extent, indicates that original trace elements concentrations, except for i.e. Sr, Cs, and Ba, could be preserved during low-temperature alteration (spilitization). Meanwhile, geochemical fingerprint of the volcanics (i.e. humped-shaped mantle normalized trace element diagrams and positive Zr–Hf anomaly) points to the crustal contamination during magma evolution, combined with the mantle metasomatism in the source via subduction-derived components (i.e. fluids), as shown by i.e. low Nb/Th and Nb/LREE ratios.

Geochemistry and tectonic setting of Middle Ordovician MORB-like basalts in the Kunlun Orogen: implications for a back-arc environment

The Central Kunlun ophiolitic melange zone, northern Tibetan Plateau, represents a remnant of the Early Paleozoic oceanic lithosphere. The Middle Ordovician Longshigeng basalts (LBs) are located in south of this melange zone and have important significance for reconstructing the opening and subduction processes of the Proto-Tethys Ocean. This study presents petrological, whole-rock geochemical, and zircon U-Pb data for these basalts. LA-ICP-MS U-Pb zircon data indicate that these basalts formed ca. 463 Ma. Geochemically, the basalts are characterized by low SiO2 (47.00–50.63 wt%) and K2O (0.22–1.05 wt%), intermediate Mg# (47–57) and high TiO2 (1.51–2.72 wt%), and are identified as tholeiitic basalts. Their chondrite-normalized REE patterns feature slight enrichments in LREEs (LREE/HREE = 2.11–3.08) with mild Eu anomalies (δEu = 0.89–1.06), and their patterns are similar to the reference line of enriched mid-ocean ridge basalt (EMORB). The primitive mantle-normalized trace element diagrams also show similarities with EMORB and are characterized by overall enrichments in large ion lithophile and high field strength elements, without notable Nb, Ta, and Ti depletions. Combining these data with the tectonic discrimination diagrams, we infer that these rocks formed in a back-arc environment in response to the northward subduction of the Proto-Tethys Ocean (South Kunlun Ocean) in the East Kunlun region, northern Tibetan Plateau.

Geochemistry of late paleoproterozoic Anjana and Amet granites of the Aravalli craton with affinities to sanukitoid series granitoids: Implications for petrogenetic and geodynamic processes

The Mangalwar Complex of the Aravalli craton is marked by the presence of late Paleoproterozoic granites referred to as Anjana Granite and Amet Granite. These granites occur as 1.64 Ga old plutons intruding greenstone sequences and migmatitic gneisses of Mangalwar Complex which comprises parts of BGC of the Aravalli craton. In the present contribution major, trace and REE data of these granites along with associated microgranular mafic enclaves (MMEs) are presented and discussed. Geochemically these granites are quartz monzonite, metaluminous, sub-alkaline and high-K calc-alkaline rocks. The most important characteristics of Anjana and Amet granites are low SiO2, high MgO, Mg#, K2O, Ba, and low Na2O/K2O ratios. In addition, the REEs show moderate to high fractionation, with (La/Yb) ratios up to 22 and 23 of the Anjana and Amet granites respectively, with no or positive europium anomalies. In the primitive mantle-normalized trace element diagrams both granites show depletion in high-field strength elements (HFSE) such as Nb, Ta, P, Ti and enrichment in LILEs. Most of these features are comparable to those of sanukitoid series rocks. Geochemically both granites are distinguished as high-Ti sanukitoids. Geochemical characteristics of MMEs suggest that they are similar to Anjana and Amet granites and in turn to sanukitoids with lower SiO2 content. They display LREE enriched patterns with low values (avg. 13) of (La/Yb)N, negative Eu anomalies and high HREE contents (58 ppm). It is suggested that the parental magma of Anjana and Amet granitic plutons originated through a four stage process (1) Generation of magmatic melts produced by partial melting of terrigeneous sediments of subducting slab in an arc setting; (2) interaction of those melts with the overlying mantle wedge, and total consumption of slab-derived melts during the reaction resulting in production of a metasomatized mantle; (3) tectonothermal event, possibly related to the slab break-off, causing asthenospheric mantle upwelling. This may have induced the melting of the metasomatized mantle and the generation of sanukitoid magmas. The parental magmas of Anjana and Amet granites and their mafic enclaves were generated at lower and higher lithospheric levels respectively (4) Granitic magma ascended due to viscosity and gravity instabilities and interacted with enclave magma at higher mantle level. Both magmas ascended towards upper crust and evolved through fractional crystallisation. Existing data suggest that in the Mangalwar Complex, the formation of sanukitoid magma started even during Mesoarchaean times and continued till late Paleoproterozoic. Formation of sanukitoid magma during this time indicates that in northern Indian shield the multi-stage subduction- accretionary orogenic processes continued for a protracted geological period and played a major role in the origin and evolution of early continental crust.

Geology and geochronology of the Jurassic magmatic arc in the Magdalena quadrangle, north-central Sonora, Mexico

This work reports on the geology and U–Pb LA-ICPMS zircon geochronology of a crustal section that is part of the Jurassic magmatic arc in the Magdalena quadrangle of north-central Sonora, Mexico. This rock succession is variably metamorphosed and strained as it was affected by Late Cretaceous shortening, intruded by early Tertiary granitoids, and further exhumed in the lower plate of the early Miocene Magdalena metamorphic core complex. The older and more extensively exposed Jurassic unit is the >3.5 km thick Sierra Guacomea rhyolite that is composed of massive to poorly bedded rhyolite, bedded quartz-phyric rhyolitic ignimbrite and interbedded ash-fall tuffs and quartz-rich sandstone beds. Three rhyolite samples collected at different localities of its outcrops yielded concordia ages of 175.2 ± 0.9, 171.7 ± 0.6, and 171.4 ± 0.7 Ma. The quartz-phyric Rancho La Víbora, Los Vallecitos, and the Agua Caliente rhyolitic domes that are associated with the Sierra Guacomea rhyolite yield concordia ages of 176 ± 0.8, 174.4 ± 0.9 and 173.1 ± 0.8 Ma, respectively. The Rancho Los Pozos unit composed of interbedded rhyolitic ash-fall tuff and flows, sandstone, siltstone and subordinate limestone beds has an estimated thickness of 600 m and yielded a crystallization concordia age of 170.7 ± 0.6 Ma from a rhyolite bed. The porphyritic El Rincón granite that intrudes into the Sierra Guacomea rhyolite yields crystallization ages of 167.43 ± 0.42 and 164.4 ± 0.7 in samples from different localities. The La Jojoba metasandstone that consists of foliated, quartz-rich to arkosic strata of fluvial origin is at least 900 m thick; detrital zircon grains dated from three sandstone samples yielded dominantly Jurassic ages with peaks at 172, 170, and 163.7 Ma, and a combined maximum depositional age of ca. 163 Ma.The younger plutons are the porphyritic El Nopalito granite that has an interpreted crystallization age of 160.8 ± 0.6 Ma, and the leucocratic, two-mica, garnet-bearing La Cebolla granite that yielded a concordia age of 158.1 ± 1 Ma. These granitic intrusions record the waning magmatic pulses of the arc, in the study quadrangle, but their volcanic equivalents were not identified. Inherited zircon grains in the reported volcanic and plutonic units are only of Jurassic age, except by two Proterozoic zircon grains yielded by the El Nopalito granite. The El Salto granite augen gneiss is a xenolith dated at 1071.9 ± 5 Ma that indicates the presence of Grenvillian basement in the study area.Major- and trace-element geochemical data indicate that the volcanic and plutonic units are silica-rich, mostly high K calc-alkaline to shoshonitic rocks associated with a continental margin arc setting. The plutons are mostly peraluminous, and in conjunction with trace element geochemistry data, they suggest crustal assimilation of magmas emplaced in a probably thickened continental crust. Chondrite-normalized REE patterns and primitive mantle-normalized trace element diagrams also suggest partial melting and fractional crystallization processes.The ages obtained indicate that the arc in the study area developed from ca. 176 to 158 Ma, encompassing a 17 m.y. interval of magmatism and associated sedimentation. Regional correlation and geochronologic published data indicate that the arc crustal section of the Magdalena quadrangle is part of the Jurassic magmatic arc that regionally lasted from ca. 190 to 158 Ma.

Late Silurian to Early Devonian volcanics in the East Kunlun orogen, northern Tibetan Plateau: Record of postcollisional magmatism related to the evolution of the Proto-Tethys Ocean

The late Early Paleozoic volcanic rocks in the East Kunlun orogen are crucial to determining collisional processes related to the evolution of the Proto-Tethys Ocean. We report new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd isotopes from these volcanic rocks and constrain their tectonic setting and magmatic processes. This volcanic suite consists mainly of foliated metarhyolites and minor metabasalts. Zircon U-Pb ages indicate that the bimodal volcanic rocks formed from Late Silurian to Early Devonian (ca. 420−409 Ma). The basalts show tholeiitic geochemical signatures characterized by low contents of SiO2 (47.19–54.83 %), MgO (2.21–7.52 %), and K2O/Na2O ratios (0.32−0.77%) and high contents of TiO2 (1.80–2.91 %). Their chondrite-normalized REE patterns are characterized by enrichments in LREEs (LREE/HREE = 3.68–6.09) with slight Eu anomalies (δEu = 0.67–1.00); their patterns are similar to the reference line of oceanic island basalt (OIB). The primitive mantle-normalized trace element diagram also shows similarities with OIBs and features overall enrichments in large ion lithophile and high field strength elements, except for Nb and Ta. The εNd(t = 422 Ma) values range narrowly from −1.79 to +1.32. These features suggest that the basalts were most likely derived from an asthenospheric mantle that was contaminated by small volumes of subcontinental lithospheric mantle (SCLM)/crust. The variations in major and trace elements show that the basalts experienced fractional crystallization of olivine, pyroxene and plagioclase. In contrast, the metarhyolites have high SiO2 (65.85–70.83 %), Na2O + K2O (6.71–10.09 %), K2O/Na2O ratios (1.69–2.80 %) and low MgO, Ni, and Cr contents. Their trace element signatures show enrichments in LILEs and LREEs (e.g. Cs, Rb, Ba), depletion of HFSEs (e.g. Nb, Ti, Ta) and high negative Eu anomalies. These features suggest a crustal origin lacking interactions with mantle melts. Considering all the geochemical and tectonic events of this area, it is concluded that the Maoniushan Formation volcanic rocks in EKOB formed in a postcollisional extensional setting related to asthenospheric mantle upwelling and continental delamination processes during the Late Silurian to Early Devonian. These tectonic-magmatic events, together with previous data, suggest that the Proto-Tethys Ocean had closed and evolved to postcollisional collapse stage since the Late Silurian.

Early Cretaceous bimodal volcanic rocks in the Yinshan belt, North China Craton: age, petrogenesis, and geological significance

Determining the age and petrogenesis of the Mesozoic magmatic rocks may provide crucial evidence regarding the tectonic setting and evolution of the North China Craton (NCC). In this paper, we present new zircon U–Pb ages and geochemical data for bimodal volcanic rocks from Zhuozi area in the eastern Yinshan belt, NCC, to discuss their petrogenesis and tectonic implications. The volcanic rock suite consists of basalts, rhyolites, and trachytes. According to the results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rhyolitic and trachytic rocks have zircon U–Pb ages of 136.9 ± 0.8 Ma and 134.0 ± 1.7 Ma, respectively, representing their eruption ages. The basalts are enriched in large-ion lithophile elements (LILE; Ba, Rb, Sr, and Th) and light rare-earth elements (LREE), and are relatively depleted in high field strength elements (HFSE; Nb, Ta, and Ti). In addition, the basalts are characterized by high fractionations of rare-earth elements (REE), with (La/Yb)N = 24.87–42.85. These features suggest that they were derived from low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related components. The rhyolitic rocks have low mean Al2O3 (12.26 wt%) and TiO2 (0.14 wt%) contents and Mg# (14.92) values, with relatively low total REE abundances. They have high alkali and Zr, Ce, Y, and Nb contents and high Ga/Al ratios, with notably positive Pb and negative Sr, Ba, and Eu anomalies observed in the primitive mantle-normalized trace element diagram. These features indicate that the rocks exhibit characteristics of A1-type granite and likely derived from the partial melting of a crustal source. The trachytic rocks are show moderate trace elements and SiO2, MgO, Fe2O3T, and TiO2 contents. They were possibly generated by fractional crystallization of the underplating basaltic magmas and assimilated minor crustal materials. The geochemical characteristics, in combination with the regional geology, suggest that the bimodal volcanic rocks formed in an intraplate extensional setting likely associated with the retreat of the subducted Paleo-Pacific plate.

Petrogenesis and tectonic implications of the Neoproterozoic adakitic and A-type granitoids in the southern Arabian-Nubian shield

Both adakitic and A-type granitoid have been recently identified in the southern ANS. The Emba-Derho and Koka granitoids yielded the same zircon U–Pb ages of 851 ± 14 Ma and 851 ± 2 Ma, respectively. The Emba-Derho granitoids show high and a narrow range of SiO2 (71.3–73.6 wt.%) and Al2O3 (14.9–15.9 wt.%) contents, but relatively lower K2O + Na2O (4.8–5.5 wt.%). They are characterized by high Sr (306–367 ppm) and very low Y (3.57–6.03 ppm) and Yb (0.38–0.61 ppm), and high Sr/Y (42–70) ratios, implying they have adakitic signatures. Their chondrite-normalized REE patterns are highly fractionated with high La/YbN (14.03 to 29.82) ratio without significant Eu anomalies (Eu/Eu* = 0.89–1.07). Their primitive mantle-normalized trace element diagram is characterized by enrichment of LILE and depletion of HREE (Ta, Nb, and Ti). The samples have low initial 87Sr/86Sr (0.70084 to 0.70263), higher positive ƐNd (+ 5.0 to + 8.2), and more radiogenic Pb isotopes. These lines of geochemical evidence indicate that the Emba-Derho granitoids were generated by partial melting of a subducted oceanic slab at the stability field of garnet. In contrast, the Koka granitoids display wide range of SiO2 (67.9–78.4 wt.%), Al2O3 (11.05–16.51 wt.%), and K2O + Na2O (5.86–8.76%) contents. The rocks have extremely low initial 87Sr/86Sr (0.68685 to 0.70099) and high ƐNd (+ 5.43 to + 5.78) and relatively less radiogenic Pb isotopic compositions compared with the Emba-Derho granitoids. They also show A2-type geochemical characteristics, suggesting that the Koka granitoids were originated from the juvenile continental crust. Both the adakitic and A-type granitoids in this study may have been formed in an arc-back-arc setting resulted from NW dipping subduction of the oceanic slab.

Petrogenesis of the Dasuji porphyry Mo deposit at the northern margin of North China Craton: Constrains from geochronology, geochemistry and isotopes characteristics

The Dasuji porphyry Mo deposit is located at the northern margin of North China Craton. Mo mineralization occurs mainly within the intrusions of alkali feldspar granite, granite porphyry, and quartz porphyry, which yield weighted mean 206Pb/238U ages (2σ) of 227.6 ± 1.5, 225.7 ± 1.5, and 224.3 ± 1.8 Ma, respectively. These ages are indistinguishable to the molybdenite ReOs isochron age of 223.9 ± 1.5 Ma (2σ). The Dasuji granites show the petrological, mineralogical, geochemical, and geotectonic characteristics of A-type granites. They exhibit low negative εNd(t) values (−16.7 to −12.6) with corresponding TDM2 (Nd) ages of 1705–2000 Ma, variable but mainly negative εHf(t) values (mostly −21.0 to −7.9) with corresponding TDM2 (Hf) ages of 1715–2728 Ma, and Pb isotopic compositions indicating that the primary magmas were dominantly derived from partial melting of ancient lower crust. In chondrite- and mantle-normalized trace element diagrams, all the granitic rocks show negative anomalies in Ba, Sr, and Eu, and a positive anomaly in Pb. The ratios of LREE/HREE, and (La/Yb)N and δEu values decrease from the alkali feldspar granite to the granite porphyry and quartz porphyry. These data are attributed to fractional crystallization processes. Fractionation of feldspar, biotite, and possibly K-feldspar produced the variations in major elements and Rb, Sr and Ba, and the distribution of REE are controlled mainly by fractionation of zircon, allanite, minor apatite, and possibly a small quantity of monazite. Together with the regional geology, the data infer that the Dasuji porphyry Mo deposit formed in a post-collision extensional setting during the Late Triassic.

Petrogenesis of the Darvazeh mafic-intermediate intrusive bodies, Qorveh, Sanandaj-Sirjanzone, Iran

The Darvazeh intrusion in the northern Sanandaj-Sirjan zone in the Alpine-Himalayan orogenic belt is composed of olivine gabbro, gabbroic diorite and granitoid units that intruded Jurassic metamorphic rocks. Field relations, textures, and whole rock geochemistry indicate that most of the mafic to intermediate composition rocks are not cumulates and instead represent liquid compositions. The tholeiitic (or ferroan) gabbros and magnesian gabbroic diorites evolved along liquid lines of descent and have different parental magmas. This conclusion is based on separate trends, often with overlapping MgO contents, for Si, Fe, Ti, Ni, Cr, Zn, Cu and Sr. The mineralogy (late versus early magnetite and contrasting proportions of magmatic amphibole) of the two suites are also slightly different as is the trace element patterns (for example Th, U, Ti and Sr). The Mg# of mafic-intermediate rocks ranges from 46 to 73, implying that the ultimate source for the most mafic rocks was in the mantle, not the crust. Flat HREE patterns, Sm/Yb ratios (<2) show that melting occurred in the shallow mantle, above the stability field of garnet. Small negative Nb-Ta-Ti anomalies and enrichments of Cs, Rb, Pb, Pb, and Sr on mantle-normalized trace element diagrams are consistent with a subduction zone source for both the tholeiitic gabbros and the magnesian gabbroic diorites. Hence, we conclude that the parental mafic magmas were generated by partial melting of spinel peridotite in the mantle wedge above a subducting slab of oceanic lithosphere. However, the composition of plagioclase-olivine pairs suggest the gabbroic magmas were not water-rich, and their high Fe/Mg and Ti/V ratios show they were relatively reduced compared to typical arc magmas. For the gabbros, melting may have been initiated by decompression in an intra-arc extensional environment caused by slab roll back or oblique subduction that partially separated the Sanandaj-Sirjan zone from the Central Iran block. Components from the subduction fluid are smaller than in the subsequent gabbroic diorites which have more typical arc compositions. These mafic magmas rose and fractionated on different paths controlled by water content and fO2, with the gabbros following reduced Fe-enrichment trends and the magnesian gabbroic diorites following more oxidized trends. The country rocks, plate reconstructions, and trace element systematics (including high Th/La ratios) show that the arc was underlain by continental crust that was assimilated into the mafic and intermediate magmas. Thus, the Darvazeh pluton reveals the complex nature of mafic intrusive rocks, which formed in a continental arc experiencing extension as the Neo-Tethys oceanic lithosphere subducted beneath the Central Iranian plate.

Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane, south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting

Post-collisional ultrapotassic magmatic rocks (15.2–18.8Ma), containing mantle xenoliths, are extensively distributed in the Sailipu volcanic field of the Lhasa terrane in south Tibet. They could be subdivided into high-MgO and low-MgO subgroups based on their petrological and geochemical characteristics. The high-MgO subgroup has olivine-I (Fo87–92), phlogopite and clinopyroxene as phenocryst phases, while the low-MgO subgroup consists mainly of phlogopite, clinopyroxene and olivine-II (Fo77–89). These ultrapotassic magmatic rocks have high MgO (4.6–14.5wt%), Ni (145–346ppm), Cr (289–610ppm) contents, and display enrichment in light rare earth element (REE) over heavy REE and enriched large ion lithophile elements (LILE) relative to high field strength elements (HFSE) with strongly negative Nb-Ta-Ti anomalies in primitive mantle-normalized trace element diagrams. They have extremely radiogenic (87Sr/86Sr)i (0.7167–0.7274) and unradiogenic (143Nd/144Nd)i (0.5118–0.5120), high (207Pb/204Pb)i (15.740–15.816) and (208Pb/204Pb)i (39.661–39.827) at a given (206Pb/204Pb)i (18.363–18.790) with high δ18O values (7.3–9.7‰). Strongly linear correlations between depleted mid-ocean ridge basalt-source mantle (DMM) and the Indian continental crust (HHCS) in Sr-Nd-Pb-O isotopic diagrams indicate that the geochemical features could result from reaction between mantle peridotite and enriched components (fluids and melts) released by the eclogitized Indian continental crust (HHCS) in the mantle wedge. The high-MgO (13.7–14.5wt%) subgroup displays higher (143Nd/144Nd)i, lower (87Sr/86Sr)i and (206Pb/204Pb)i ratios and lower δ18O values compared with the low-MgO (4.6–8.8wt%) subgroup. High Ni (850–4862ppm) contents of olivine phenocrysts and high whole-rock SiO2, NiO, low CaO contents indicate that the low-MgO ultrapotassic magmatic rocks are derived from partial melting of olivine-poor mantle pyroxenite. However, lower Ni concentrations of olivine phenocryst and lower whole-rock SiO2, NiO, higher CaO contents of the high-MgO ultrapotassic rocks may indicate their peridotite mantle source. This could be attributed to different amounts of silicate-rich components added into the mantle sources of the parental magmas in the mantle wedge caused by the northward subduction of the Indian continental lithosphere. The reaction-formed websterite xenoliths, reported for the first time in this study, are made up of anhedral and interlocking clinopyroxene (45–65vol%) and orthopyroxene (30–50vol%) with minor phlogopite (<3vol%) and quartz (<2vol%) and are suggested to be formed by silicate metasomatism of the mantle peridotite. The harzburgites, another major type of mantle xenolith in south Tibet, have a mineral assemblage of olivine (60–75vol%), orthopyroxene (20–35vol%), clinopyroxene (<3vol%), phlogopite (<2vol%) and spinel (<2vol%) and may have experienced subduction-related metasomatism. Combined with two types of ultrapotassic magmas, we propose that compositions of mantle wedge beneath south Tibet may gradually evolve from harzburgite through lherzolite to websterite with strong metasomatism of silicate-rich components in their mantle source region. Partial melting of the enriched mantle sources could be triggered by rollback of Indian continental slab during 25–8Ma in south Tibet.

Petrogenesis of the Mesozoic Shuikoushan peraluminous I-type granodioritic intrusion in Hunan Province, South China: Middle–lower crustal reworking in an extensional tectonic setting

The Mesozoic Shuikoushan granitoid intrusion in the Jiangnan orogen between the Yangtze and the Cathaysia blocks in southern Hunan Province, South China, provides an opportunity to probe deep-crustal composition and structure, and to investigate the geodynamic evolution of South China. The intrusion is composed of granodiorite with I-type geochemical affinities and contains zircon with an U–Pb concordant age of 158.3±1.2Ma. Rocks from the intrusion have variable SiO2 from 58.4wt.% to 65.2wt.%, Al2O3 from 15.0wt.% to 17.2wt.%, TiO2 from 0.57wt.% to 0.76wt.%, and P2O5 from 0.27wt.% to 0.52wt.%. They have A/CNK from 0.97 to 1.23 (1.04 on average) and are dominantly peraluminous. They are characterized by enrichment in LREE ([La/Sm]N=3.47–5.19) and LIL (e.g., K, Rb) and depletion in HFSE (e.g., U, Th) in primitive mantle normalized trace element diagrams. These geochemical features indicate that the rocks are I-type granodiorites and have undergone extensive fractional crystallization of hornblende, biotite, titanite, and apatite. All samples show negative εHf(t) (−10.6 to −8.1) and εNd(t) (−5.92 to −6.13) values, with relatively high initial 87Sr/86Sr (ISr) (0.7101–0.7102) and δ18O ratios (8.4–9.7‰). These values significantly differ from mantle values, suggesting that they are generated by the amphibole-dehydration melting of a mafic source in the middle-to-lower crust beneath the Jiangnan orogen. The emplacement of the Shuikoushan granitoid intrusion is contemporaneous with the late Jurassic to early Cretaceous extension within the Shi-Hang rift zone, indicating that it may have been formed under an extensional regime in response to both the Pacific superplume activity and westward subduction of the Pacific Plate. The extensional processes along with asthenosphere upwelling and basaltic underplating may have triggered the widespread late Jurassic granitoid intrusion in southern Hunan Province and possibly in South China.

Apatite geochemistry of the Taihe layered intrusion, SW China: Implications for the magmatic differentiation and the origin of apatite-rich Fe-Ti oxide ores

The Taihe intrusion is one of the layered intrusions situated in the central zone of the Emeishan Large Igneous Province (ELIP), SW China. The cyclic units in the Middle Zone of the intrusion are composed of apatite-magnetite clinopyroxenite at the base and gabbro at the top. The apatite-rich oxide ores contain 6–12modal% apatite and 20–50modal% Fe-Ti oxides evidently distinguished from the coeval intrusions in which apatite-rich rocks are poor in Fe-Ti oxides. Most of apatites of the Taihe Middle and Upper Zones are fluorapatite, although four samples show slightly high Cl content in apatite suggesting that they crystallize from a hydrous parental magma. Compared to the apatite from the gabbro of the Panzhihua intrusion, situated 100km to the south of the Taihe intrusion, the apatite of the Taihe rocks is richer in Sr and depleted in HREE relative to LREE. The calculated magma in equilibrium with apatite of the Taihe Middle and Upper Zones also shows weakly negative Sr anomalies in primitive mantle normalized trace element diagrams. These features indicate that the apatite of the Taihe Middle and Upper Zones crystallizes after clinopyroxene and before plagioclase. The apatite of the Taihe Middle and Upper Zones shows weakly negative Eu anomalies suggesting a high oxygen fugacity condition. The high iron and titanium contents in the oxidizing magma result in crystallization of Fe-Ti oxides. Crystallization of abundant Fe-Ti oxides and clinopyroxenes lowers the solubility of phosphorus and elevates SiO2 concentration in the magma triggering the saturation of apatite. The positive correlations of Sr, V, total REE contents and Ce/Yb ratio in apatite with cumulus clinopyroxene demonstrate approximately compositional equilibrium between these phases suggesting they crystallized from the same ferrobasaltic magma. Early crystallization and accumulation of Fe-Ti oxide together with apatite produced the apatite-rich oxide ores at the base of the cyclic units of the Taihe Middle Zone.

Mid-Neoproterozoic Tadong amphibolites at the junction of the East Kunlun and Western Qinling Orogens – a record of continental rifting during the break-up of Rodinia

ABSTRACTMid-Neoproterozoic rift-related Tadong amphibolites (TDA) are situated in the junction of the East Kunlun Orogen (EKO) and the West Qinling Orogen (WQO) in western China, and have important significance in the break-up of the Supercontinent Rodinia. The mineral assemblage of the TDA exhibits amphibolite facies metamorphism, and the protolith is basic volcanic rocks. Here, we present a detailed laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb age and geochemical data of TDA. LA-ICP-MS U–Pb zircon age determination indicates that the basic volcanics formed at ca. 743–711 Ma. Geochemically, TDAs are characterized by low SiO2 (46.06–50.18%), K2O (0.66–1.86%), and Mg# (40–45) and high TiO2 (1.9–3.5%), and are attributed to tholeiite basalts. Their chondrite-normalized REE patterns are characterized by moderate enrichments in LREEs (LREE/HREE = 4.98–5.51) with insignificant Eu anomalies (δEu = 0.99–1.12). The primitive mantle-normalized trace element diagrams are characterized by pronounced enrichment of high-field-strength elements (HFSEs, e.g. REE, Nb, Ta, Zr, Hf) with slightly positive anomalies of Ti, which is different from the subduction zone basalts and normal mid-ocean ridge basalts (NMORBs), but similar to the ocean-island basalts (OIBs) and Emeishan continental flood basalts (ECFBs), resembling the features of an OIB-like mantle source. Most samples exhibit high Nb/Ta (16.6–17.4), Nb/La (0.70–0.94), and positive ΔNb (ΔNb = 1.74 + log (Nb/Y) − 1.92*log (Zr/Y)) values. These geochemical features suggest that the Tadong basaltic rocks were most likely derived from an OIB-like mantle source and were the products of a lower degree partial melting of garnet lherzolite. Along with the tectonic discrimination diagrams, it is further inferred that these rocks were formed in a continental rifting setting. These results from our research confirm the existence of continental rifting in the junction of the EKO and the WQO in the mid-late Neoproterozoic, implying that the rifting is a response to the break-up of the Supercontinent Rodinia.

Late Cenozoic calc-alkaline volcanism over the Payenia shallow subduction zone, South-Central Andean back-arc (34°30′–37°S), Argentina

A series of mesosilicic volcanic centers have been studied on the San Rafael Block (SRB), 300 km to the east of the present-day volcanic arc. K-Ar ages indicate that this magmatic activity was developed in at least two stages: the older volcanic centers (∼15–10 Ma) are located in the central and westernmost part of the SRB (around 36°S and 69°W) and the younger centers (8–3.5 Ma) are located in an eastern position (around 36°S and 69°30′W) with respect to the older group. These volcanic rocks have andesitic to dacitic compositions and correspond to a high-K calc-alkaline sequence as shown by their SiO2, K2O and FeO/MgO contents. Elevated Ba/La, Ba/Ta and La/Ta ratios show an arc-like signature, and primitive mantle normalized trace element diagrams show typical depletions of high field strength elements (HFSE) relative to large ion lithophile elements (LILE). Rare earth element (REE) patterns suggest pyroxene and amphibole crystallization. Geochemical data obtained for SRB volcanic rocks support the proposal for a shallow subduction zone for the latest Miocene between 34°30″–37°S. Regionally, SRB volcanism is associated with a mid-Miocene to early Pliocene eastward arc migration caused by the shallowing of the subducting slab in the South-Central Andes at these latitudes, which represents the evolution of the Payenia shallow subduction segment. Overall, middle Miocene to early Pliocene volcanism located in the Payenia back-arc shows evidence for the influence of slab-related components. The younger (8–3.5 Ma) San Rafael volcanic rocks indicate the maximum slab shallowing and the easternmost extent of slab influence in the back-arc.

Zircon U–Pb age and Sr–Nd–Hf–O isotope geochemistry of the Paleocene–Eocene igneous rocks in western Gangdese: Evidence for the timing of Neo-Tethyan slab breakoff

Northward Neo-Tethyan oceanic lithosphere subduction beneath southern Tibet in the Mesozoic–Early Cenozoic produced continental arc magmas in the ~1600km-long Gangdese belt. The most voluminous magmatism occurred in the Paleocene–Eocene, and is characterized by extensive I-type calc-alkaline to high-K calc-alkaline Linzizong volcanic rocks, and coeval plutons. These rocks have been extensively studied in the eastern Gangdese belt (east of ∼89°E), but few data exist from the western Gangdese belt. New data for eleven samples of these rocks, combined with existing data from the literature, show that they are similar to the eastern Gangdese belt rocks, with relative depletions in Nb, Ta, P, and Ti, and enrichments in Rb, Ba, Th, U, K, Pb, Zr, and Hf on a primitive mantle-normalized trace element diagrams, typical of continental arc-related igneous rocks. However, compared to the east, western Gangdese igneous rocks range to higher K2O contents (up to 6.1wt.%), higher (87Sr/86Sr)i ratios (up to 0.7151), and lower εNdi values (down to −8.1), suggesting that an evolved crustal source was involved in arc magmatism.The Gangdese arc magmatism lasted to ~80Ma, and has a gap or quiescent period afterwards. Starting at ~69Ma, the arc magmatism initiated and shifted southward from ~30.5°N to ~29.5°N in southern Tibet with an abrupt change of India-Asia convergence rate. The magmas through the whole Gangdese belt at ~69–53Ma are characterized by intermediate εNdi values (−0.6 to +4.0), positive εHfi values (+3.8 to +7.1), intermediate δ18O values (+5.0‰ to +6.5‰), and low Th/Y and La/Yb ratios (<20). These magmas were likely derived from the mantle with crustal contamination by MASH process in response to Neo-Tethyan slab rollback as proposed by previous studies.The Sr–Nd–Hf–O isotopic compositions of Linzizong volcanic rocks and coeval intrusions are relatively homogeneous during ~69–53Ma, but show a wide range at ~53–50Ma. Zircon temperature values from these rocks are distinctively high during the latter period and whole rock Th/Y and La/Yb ratios increased significantly after ~50Ma. We suggest that the high temperature and heterogeneous geochemistry of these magmas at ~53–50Ma reflect Neo-Tethyan slab breakoff during India-Asia collision, which triggered asthenospheric mantle upwelling. Mantle derived magmas resulted in extensive heating, induced crustal melting, and generated magmas with variable isotopic compositions. Tibetan crust was significantly thickened after ~50Ma.

Plume-lithosphere interaction in the generation of the Tarim large igneous province, NW China: Geochronological and geochemical constraints

The magmatism in the early Permian Tarim large igneous province (TLIP) in NW China is represented by basaltic lava flows in Keping and ultramafic-mafic-felsic intrusions and mafic dikes in Bachu, northwestern Tarim Craton. This paper reports new Ar-Ar dating results and chemical compositions of Keping basalts and Bachu dikes, with aims of better characterizing the timing of and mantle/crustal contribution to the TLIP. The Keping basalts yield two well-defined ⁴⁰Ar/³⁹Ar plateau ages of 287.3 ± 4.0 Ma and 287.9 ± 4.1 Ma, which, together with age data from the literature, define a magmatic event at ∼289 Ma. The intrusions and dikes in Bachu are believed to have formed at ∼279 Ma based on screened literature data. Thus, they together define two magmatic episodes. The Keping basalts, representing the earlier episode, have alkaline affinity (SiO₂ = 44.0-47.9 wt.%, Na₂O + K₂O = 3.7-4.9 wt.%), low MgO (4.3-5.9 wt.%) and high TiO₂ (3.8-5.1 wt.%) contents, showing fractionated chondrite-normalized LREE and nearly flat HREE patterns [(La/Yb)_N = 6.27-7.71; (Dy/Yb)_N = 1.36-1.48] with noticeable negative Nb and Ta anomalies in the primitive mantle-normalized trace element diagram. They have negative and relatively uniform ε_Nd(t) (−2.3 to −3.8) and low (²⁰⁶Pb/²⁰⁴Pb)_i (17.43-17.57). We argue that these “crustal signatures” cannot be attributed to crustal assimilation because neither ε_Nd(t) nor (²⁰⁶Pb/²⁰⁴Pb)_i correlates with SiO₂; rather they are more likely derived from a sub-continental lithospheric mantle (SCLM) source metasomatized by subduction-related processes. The Bachu dikes, representing the later episode and confined to the margins of the Tarim Craton, have similar MgO (3.6-5.4 wt.%) and TiO₂ (3.1-4.7 wt.%) contents to the Keping basalts, and display more fractionated REE patterns [(La/Yb)_N = 10.1-14.0; (Dy/Yb)_N = 1.79-1.99]. They have variable isotope compositions [ε_Nd(t) = −0.3-4.8, (²⁰⁶Pb/²⁰⁴Pb)_i = 17.50-18.11] and display OIB-like trace element signatures. Correlations between isotopic and trace element ratios indicate that some dikes with low ε_Nd(t) and low initial Pb isotope ratios could have been subjected to crustal assimilation. We propose a model involving plume-lithosphere interaction to account for the two discrete magmatic episodes with distinct mantle sources in the TLIP. The earlier episode was formed in response to the impact of a sub-lithospheric mantle plume at the base of the SCLM. Partial melting of the metasomatized lithospheric mantle was triggered by temperature increase due to conductive heating of the impregnating mantle plume. The later episode was generated by decompression melting of the mantle plume, as a result of deflection of the plume towards the margins of the Tarim Craton with thinner lithosphere.

First Evidence of Lamprophyric Magmatism from the Konya Region, Turkey: a Genetic Link to High‐K Volcanism

AbstractIn the vicinity of Konya (Turkey), mafic, micro‐porphyritic sub‐volcanic rocks intrude into the Mesozoic units, which represents the only example of such a rock type in the region. 40Ar/39Ar dating of two whole rock samples from the sub‐volcanics gave ages of 13.72±0.13 and 12.40±0.11 Ma, suggesting temporal association to the Late Miocene‐Pliocene high‐K calc‐alkaline volcanism in the region. The mineral chemistry and geochemical data permit us to classify the rocks as “minette” lamprophyres. They include diopside and phlogopite phenocrysts in a microcrystalline groundmass composed of sanidine, phlogopite, diopside and titano‐magnetite. Segregation and ocelli‐like globular structures occur commonly in the samples. In terms of major elements, the lamprophyres are calcalkaline, and potassic to ultrapotassic rocks. All the lamprophyres display strong enrichments in LILE (Rb, Ba, K, Sr), radiogenic elements (Th, U) and LREE (La, Ce) and prominent negative Nb, Ta, and Ti anomalies on primordial mantle‐normalized trace element diagrams. Geochemical data suggest that the lamprophyres and high‐K calc‐alkaline rocks in the region derived from a subduction‐modified lithospheric mantle source affected by different metasomatic events. Lamprophyric magmatism sourced phlogopite‐bearing veins generated by sediment‐related metasomatism via subduction, but high‐K calc‐alkaline magmas are possibly derived from a mantle source affected by fluid‐rich metasomatism.

Geochronological, geochemical and Sr–Nd–Hf isotopic constraints on the origin of the Cretaceous intraplate volcanism in West Qinling, Central China: Implications for asthenosphere–lithosphere interaction

This paper reports new zircon LA-ICP-MS UPb age and Lu-Hf isotopes, laser-fusion 40Ar/39Ar age, mineral chemistry, bulk-rock major and trace element, and SrNd isotope data from intraplate volcanic rocks in the Duofuntun Area, West Qinling. It was dated at 105.8±0.9Ma (UPb Zircon) and 103±2Ma (matrix glass laser-fusion 40Ar/39Ar dating). The Duofuntun Volcanic Rocks (DVR) are dominantly composed of alkali basalts with minor basanites and basaltic andesites. Some evolved samples of the DVR underwent a variable fractionation of olivine, clinopyroxene, plagioclase, FeTi oxide, Cr-rich spinel and apatite. Chondrite-normalized rare earth element and primitive mantle-normalized trace element patterns of the DVR remarkably resemble those of oceanic island basalts (OIB). Nearly all the alkali basalts show positive Nb, Ta and Ti anomalies and negative Rb, Th and P anomalies, whereas basanites do not display positive Ti anomalies. Nearly all the samples show marked negative Rb and K anomalies relative to their neighbouring elements on a primitive mantle normalized trace element diagram. 87Sr/86Sr(i) and 143Nd/144Nd(i) ratios for all the mafic samples also show an OIB affinity, covering a range of 0.703264–0.705032 and 0.512718–0.512873, respectively. Several lines of evidence indicate that both garnet and amphibole play a key role for the genesis of the DVR. The melt segregation PT conditions in the source region, around 1371°C at 1.98–3.25GPa, combined with trace element collectively suggest a polybaric melting process during the formation of the DVR. The non-modal batch partial melting modelling result indicates the DVR were generated by small-degree polybaric partial melting of amphibole-bearing spinel peridotites (ca. 4%) and amphibole-bearing garnet peridotites (ca. 0.7%). We suggest the DVR were not formed in a plume. Rather, they originated by the interaction between the upwelling asthenosphere, which might provide both materials and heat flux, and hydrous mineral-bearing (amphibole) lithosphere during lithospheric extension.

Genesis of the Chehugou Mo-bearing granitic complex on the northern margin of the North China Craton: geochemistry, zircon U–Pb age and Sr–Nd–Pb isotopes

AbstractThe Chehugou granite-hosted molybdenum deposit is typical of the Xilamulun metallogenic belt, which is an important Mo–Ag–Pb–Zn producer in China. A combination of major and trace element, Sr and Nd isotope, and zircon U–Pb isotopic data are reported for the Chehugou batholith to constrain its petrogenesis and Mo mineralization. The zircon SIMS U–Pb dating yields mean ages of 384.7 ± 4.0 Ma and 373.1 ± 5.9 Ma for monzogranite and syenogranite and 265.6 ± 3.5 Ma and 245.1 ± 4.4 Ma for syenogranite porphyry and granite porphyry, respectively. The Devonian granites are calc-alkaline with K2O/Na2O ratios of 0.44–0.52, the Permian granites are alkali-calcic with K2O/Na2O ratios of 1.13–1.25, and the Triassic granites are calc-alkaline and alkali-calcic rocks with K2O/Na2O ratios of 0.78–1.63. They are all enriched in large-ion lithophile elements (LILEs) and depleted in high-field-strength elements (HFSEs) with negative Nb and Ta anomalies in primitive mantle-normalized trace element diagrams. They have relatively high Sr (189–1256 ppm) and low Y (3.87–5.43 ppm) concentrations. The Devonian granites have relatively high initial Sr isotope ratios of 0.7100–0.7126, negative ɛNd(t) values of −12.3 to −12.4 and 206Pb/204Pb ratios of 16.46–17.50. In contrast, the Permian and Triassic granitoids have relatively low initial 87Sr/86Sr ratios (0.7048–0.7074), negative ɛNd(t) values of −10.1 to −13.1 and 206Pb/204Pb ratios of 17.23–17.51. These geochemical features suggest that the Devonian, Permian and Triassic Chehugou granitoids were derived from ancient, garnet-bearing crustal rocks related to subduction of the Palaeo-Asian Ocean and subsequent continent–continent collision between the North China and Siberian plates.