Subsequent Fractional Crystallization Research Articles

Strontium isotopes — A persistent tracer for the recycling of Gondwana crust in the Variscan orogen

Voluminous Cambro-Ordovician quartz-rich sandstones blanket large parts of North Africa and Arabia, as well as terranes that later became separated from northern Gondwana. These sandstones are the result of widespread and intense chemical weathering that gave rise to a strong depletion in Na, Ca, and Sr and, concomitantly, very high K/Na and Rb/Sr ratios. Because of their high Rb/Sr ratios, these rocks developed through time highly radiogenic Sr-isotopic compositions (in part with 87Sr/86Sr0>1.0). This particular geochemical signature may provide a geochemical provenance indicator for early Palaeozoic sedimentary rocks, which may be used to (i) trace the fate of particular lithological units during orogenic processes and (ii) constrain the provenance of sedimentary rocks on terranes of disputed palaeogeographic position. Among the Palaeozoic sedimentary and volcanic rocks deposited on the Gondwanan shelf of Saxo-Thuringia (Germany), only Lower Ordovician siliciclastic rocks like those of the Tremadocian Frauenbach Group (Schwarzburg Anticline) are characterized by this weathering-related strong depletion in Na, Ca, and Sr. Metamorphic nappes of the adjacent Erzgebirge consist of lithologies originally deposited on the Gondwana shelf. Modeling of the Sr-isotopic composition demonstrates that the high Rb/Sr ratios accounting for the highly radiogenic measured 87Sr/86Sr values are not due to metamorphic element mobility, but represent a primary signature already acquired at the time of deposition. Thus, the “Frauenbach Sr-isotopic signature” – similar to the geochemical fingerprint – can be traced through metamorphism even to high-grade conditions. High 87Sr/86Sr ratios are also present in siliciclastic rocks derived from the erosion of the exhumed Variscan orogen. The appearance and disappearance of this signature put constraints on the erosion history of the Variscan orogen; its dilution allows a rough estimate of the relative contribution of the high-87Sr/86Sr metamorphic rocks to the erosional debris. Similarly, the high Rb/Sr granites of the Erzgebirge may not be the product of extreme fractional crystallization alone, but may reflect the involvement of protoliths with high Rb/Sr. Granites with radiogenic Sr-isotopic composition share – despite geochemical modification by differential melting of the source and subsequent fractional crystallization – geochemical fingerprints with the Frauenbach Group, in particular increased W, Sn, F, Li, and Rb contents and low Sr abundances. Should it turn out that high initial 87Sr/86Sr and high W, Sn, F, and Li of granites are linked to one particular crustal protolith, both Sn-enriched granites of Variscan Europe and Palaeozoic sedimentary rocks with Frauenbach geochemical signatures represent a Gondwana fingerprint.

Zircon U–Pb ages, Hf–O isotopes and whole-rock Sr–Nd–Pb isotopic geochemistry of granitoids in the Jinshajiang suture zone, SW China: Constraints on petrogenesis and tectonic evolution of the Paleo-Tethys Ocean

The Jinshajiang suture zone, located in the eastern part of the Tethyan tectonic domain, is noticeable for a large-scale distribution of Late Jurassic to Triassic granitoids. These granitoids were genetically related to the evolution of the Paleo-Tethys Ocean. Beiwu, Linong and Lunong granitoids occur in the middle zone of the Jinshajiang Suture Zone, and possess similar geochemical features, indicating that they share a common magma source. SIMS zircon U–Pb dating reveals that the Beiwu, Linong and Lunong granitic intrusions were emplaced at 233.9 ± 1.4 Ma (2σ), 233.1 ± 1.4 Ma (2σ) and 231.0 ± 1.6 Ma (2σ), respectively. All of these granitoids are enriched in abundances of Si (SiO 2t = 65.2–73.5 wt.%), and large-ion lithophile elements (LILEs), but depleted in high field strength elements contents (HFSEs, e.g., Nb, Ta, and Ti). In addition, they have low P 2O 5 contents (0.06–0.11 wt.%), A/CNK values ([molecular Al 2O 3/(CaO + Na 2O + K 2O)], mostly < 1.1) and 10,000 Ga/Al ratios (1.7–2.2), consistent with the characteristics of I-type granites. In terms of isotopic compositions, these granitoids have high initial 87Sr/ 86Sr ratios (0.7078–0.7148), Pb isotopic compositions [( 206Pb/ 204Pb) t = 18.213–18.598, ( 207Pb/ 204Pb) t = 15.637–15.730 and ( 208Pb/ 204Pb) t = 38.323–38.791], zircon δ 18O values (7.3‰–9.3‰) and negative ε Nd(t) values (− 5.1 to − 6.7), suggesting they were predominantly derived from the continental crust. Their Nb/Ta ratios (average value = 8.6) are consistent with those of the lower continental crust (LCC). However, variable ε Hf(t) values (− 8.6 to + 2.8) and the occurrences of mafic microgranular enclaves (MMEs) suggest that mantle-derived melts and lower crustal magmas were involved in the generation of these granitoids. Moreover, the high Pb isotopic ratios and elevated zircon δ 18O values of these rocks indicate a significant contribution of the upper crustal composition. We propose a model in which the Beiwu, Linong and Lunong granitoids were generated under a late collisional or post-collisional setting. It is possible that this collision was completed before Late Triassic. Decompression induced mantle-derived magmas to be underplated and provided the heat for the anatexis of the crust. Hybrid melts including mantle-derived and the lower crustal magmas were then generated. The hybrid melts thereafter ascended to a shallow depth and resulted in some degree of sedimentary rock assimilation. Such three-component mixing magma source and subsequent fractional crystallization could be responsible for the formation of the Beiwu, Linong and Lunong granitoids.

Late Neoproterozoic volcano-sedimentary successions of Wadi Rufaiyil, southern Sinai, Egypt: A case of transition from late- to post-collisional magmatism

The Neoproterozoic Wadi Rufaiyil volcano-sedimentary successions are exposed in southern Sinai (Egypt), at the northernmost segment of the Arabian–Nubian Shield (ANS). Two eruption phases have been recognized within these successions; both of them are intercalated with the Hammamat Clastics. The lower unit is composed predominantly of andesite to dacite and less commonly tuffs, while the upper one comprised mainly rhyolite with subordinate dacite–rhyodacite and their pyroclastics. The lower Hammamat Clastics are intermediate in compositions, contrary to the upper ones which are acidic and more mature. Geochemically, the volcanic rocks are mostly high-K calc-alkaline, corresponding to andesite through dacite–rhyodacite and rhyolite. Evolved parts of the younger phase (i.e., rhyolite) have transitional character to alkaline A-type magma, related most likely to their high fractionated nature rather than a switch in the tectonic regime. REE patterns are characterized by fractionated LREE [(La/Sm) n = 2.50–3.38] and nearly flat HREE [(Gd/Lu) n = 1.16–1.62], where the most evolved variants display prominent negative Eu anomalies (Eu/Eu * = 0.04–0.46). Dokhan Volcanics and associated Hammamat Clastics from Sinai and the Egyptian Eastern Desert (ED) share these geochemical characteristics. The Wadi Rufaiyil lavas display geochemical characteristics of both pre-collisional arc-type and post-collisional within-plate environments, suggesting eruption in a transition from late- to post-collisional tectonic setting. The integrated data point to generation by partial melting of mafic lower crust and/or upper mantle source to produce andesitic–dacitic (tonalite) magma with subsequent extensive fractional crystallization. The extrusion of the Wadi Rufaiyil volcanics was likely associated with extensive denudation of the pre-630 Ma orogenic edifice as well as the 630–600 Ma late-collisional products. Such a scenario implies delamination of thick lithosphere root during late- to post-collisional stage, facilitating the upwelling of hot asthenopsheric material and surface uplifting of northern ANS. This may have been the trigger to transition into the widespread post-collisional alkaline A-type magmatism at ∼600 Ma.

Contrasting Middle Jurassic and Early Cretaceous mafic intrusive rocks from western Liaoning, North China craton: petrogenesis and tectonic implications

AbstractZircon U–Pb dating, whole-rock major oxide, trace element and Sr–Nd isotopic data are presented for the Late Mesozoic mafic intrusive rocks from Yiwulüshan of western Liaoning along the eastern segment of the Yanshan belt, North China craton, with two episodes of magmatism documented. Middle Jurassic hornblende-rich gabbros show enrichment of large ion lithophile elements and light REE, and prominent depletion in high field strength elements, and possess moderately enriched isotopic compositions with (87Sr/86Sr)i ranging from 0.7056 to 0.7065 and ɛNd(t) from −5.0 to −7.1. These features suggest that the gabbros were derived from an amphibole-bearing harzburgitic lithospheric mantle source metasomatized recently by slab-derived fluids. By contrast, Early Cretaceous mafic dykes are gabbroic dioritic to dioritic in composition, with comparable trace element characteristics to continental crust and depleted isotopic signatures ((87Sr/86Sr)i = 0.7048–0.7055, ɛNd(t) = 0 to −3.0). They probably originated from partial melting of a relatively fertile asthenospheric mantle in the spinel stability field, with subsequent lower crustal assimilation and fractional crystallization. These two contrasting mafic intrusive suites, together with multiple Mesozoic mafic volcanic rocks from western Liaoning, documented a localized lithospheric thinning process, mainly through prolonged hydro-weakening or melt–rock interaction and triggered by gravitational collapse, possibly within an evolved post-collisional to within-plate extensional regime.

Late Permian to Early Triassic mafic to felsic intrusive rocks from North Liaoning, North China: Petrogenesis and implications for Phanerozoic continental crustal growth

Zircon U–Pb dating, whole-rock major oxide, trace element and Sr–Nd–Hf isotopic data are presented for the Late Permian to Early Triassic Shijianfang batholith from Faku in North Liaoning Province, North China. Two main magmatic suites are documented: one, with an emplacement age of ca. 260 Ma, is mafic to intermediate in composition, forming small gabbroic to monzonitic intrusions; the other is felsic and quartz monzonitic to granitic in composition, with an emplacement age of ca. 250 Ma. Geochemically, the rocks from the mafic suite show strong enrichment of LILE and LREE and depletion in HFSE, and also have moderately depleted isotopic compositions, with I Sr ranging from 0.7049 to 0.7054, ε Nd( t) from + 2.72 to − 1.82 and zircon ε Hf( t) values from + 3.4 to + 7.1. These features suggest derivation from high degree partial melting of a subduction-related metasomatized lithospheric mantle source. The rocks from the felsic suite range from 66.7 to 77.2 wt.% SiO 2, and define a continuous chemical evolutionary trend, from less-differentiated low-silica members displaying LREE enriched patterns, no Eu anomalies and high Ba, Sr, Zr and Hf, to more-differentiated, high-silica members with higher alkalis, strong depletion in Ba, Sr and Ti, enrichment in HFSE, fractionated REE patterns and strong negative Eu anomalies. However, they exhibit indistinguishable isotopic compositions from the mafic rocks, with I Sr ranging from 0.7043 to 0.7062, ε Nd( t) from + 2.28 to − 0.55 and zircon ε Hf( t) values from + 3.6 to + 6.6. This suggests that the parental magma for the felsic suite originated from partial melting of mixed protoliths composed of juvenile basaltic underplate and ancient lower crustal materials. Subsequent fractional crystallization, with overprinting by magmatic hydrothermal fluids, can explain the geochemical variations of the felsic suite. The juvenile character of both lithospheric mantle and crustal rocks, as recorded by the Shijianfang batholith, suggests that the northern Liaoning block forms part of a Phanerozoic accretionary orogenic belt. This observation indicates that the Chifeng-Kaiyuan Fault likely represents the Mesozoic lithospheric boundary between the North China Craton and the Xing-Meng Orogenic Belt in northern Liaoning. The Shijianfang batholith, together with Late Permian to Middle Triassic adakitic and A-type rocks, mafic–ultramafic cumulates and alkaline intrusions from the neighboring areas, constitutes an important post-collisional to intra-plate anorogenic magmatic province within the continental interior of the newly amalgamated North China–Mongolian plate. The unique mafic and felsic coupling in the Shijianfang batholith provides a good proxy record for the multiple-step vertical continental crustal growth in the newly amalgamated continental interior during the Phanerozoic, triggered by continued magmatic underplating due to lithospheric delamination and hot asthenospheric upwelling within a post-collisional to post-orogenic extensional regime.

Petrogenesis of highly fractionated I-type granites in the Zayu area of eastern Gangdese, Tibet: Constraints from zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes

The Cretaceous granitoids in the middle and northern Gangdese, Tibet are generally interpreted as the products of anatexis of thickened deep crust genetically associated with the Lhasa-Qiangtang collision. This paper reports bulk-rock major element, trace element and Sr-Nd isotopic data, zircon U-Pb age data, and zircon Hf isotopic data on the Zayu pluton in eastern Gangdese, Tibet. These data shed new light on the petrogenesis of the pluton. Our SHRIMP zircon U-Pb age dates, along with LA-ICPMS zircon U-Pb age dates recently reported in the literature, indicate that the Zayu pluton was emplaced at about 130 Ma, coeval with Early Cretaceous magmatic rocks in other areas of eastern Gangdese (e.g., Rawu, Baxoi areas) and the Middle Gangdese. The Zayu pluton samples lack amphibole and muscovite, and are compositionally characterized by high SiO2 (69.9%–76.8%), K2O (4.4%–5.7%), and low P2O5 (0.05%–0.12%). These samples also have A/CNK values of 1.00–1.05, and are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, P, Ti, and Eu. These geochemical features suggest that the Zayu pluton samples are metaluminous to slightly peraluminous and are of highly fractionated I-type granite. The Zayu pluton samples have high ɛNd(t) values (−10.9–−7.6) and low initial 87Sr/86Sr ratios (0.7120–0.7179) relative to melts derived from mature continental crust in the Gangdese (e.g., Ningzhong Early Jurassic strongly peraluminous granite). The Zayu pluton samples are heterogeneous in zircon ɛHf(t) values (−12.8–−2.9), yielding ancient zircon Hf crustal model ages of 1.4–2.0 Ga. The data obtained in this study together with the data in the recent literature suggest that the Early Cretaceous granitoids in eastern Gangdese represent the eastward extension of the Early Cretaceous magmatism in the middle Gangdese, and that the Lhasa micro-continent block with ancient basement may extend for ∼2000 km from east to west. Zircon Hf isotopic data and bulk-rock zircon saturation temperature (789–821 °C) indicate that mantle-derived materials likely played a role in the generation of the Zayu pluton. We propose that the Zayu pluton was most likely generated in a setting associated with southward subduction of the Bangong-Nujiang ocean floor, where mantle wedge-derived magmas may have provided the heat and material for the anatexis of ancient crust of the Lhasa micro-continent, resulted in hybrid melts (i.e., mantle-derived basaltic magmas + crust-derived felsic magmas). Such hybrid melts with subsequent fractional crystallization are responsible for the highly evolved Zayu pluton (crust thickening is not a prerequisite).

Fractional crystallization of olivine melt inclusion in shock-induced chondritic melt vein

The formation of ringwoodite, wadsleyite and majorite from their parental low-pressure polymorphs in melt veins in chondritic meteorites is usually interpreted to be the result of shock-induced solid-state phase transformation. Formation and survival of individual mineral melt enclaves in the chondritic high-pressure melt was not considered a viable possibility. We report evidence for melting of individual large olivine fragments entrained in melt veins, their survival as melt enclaves in the chondritic melts and their subsequent fractional crystallization at high-pressures and temperatures. The fractionally crystallized olivine melt enclaves appear to be ubiquitous in chondrites. In contrast, Ca-poor pyroxene fragments in the same veins and Ca-poor pyroxene in chondrules entrained do not show any sign of melting. Texture and compositions of olivine fragments are indicative of fractional crystallization from individual olivine melts alone. Fragments of original unzoned olivine (Fa 24–26) melted, and melts subsequently fractionally crystallized to Mg-rich wadsleyite (Fa 6–10) and Mg-poor ringwoodite (Fa 28–33) with a compositional gap of ≤26 mol% fayalite. In contrast, compositions of ringwoodite and wadsleyite that emerged from solid-sate phase transformations are identical to that of parental olivine thus erasing any source of enigma. The olivine monomineralic melts barely show any signs of mixing with the chondritic liquid prior to or during their individual fractional crystallization. Our findings demonstrate that the formation of high-pressure minerals during shock events in asteroids also results from melting and fractional crystallization from some individual mineral melts that barely mixed with the chondritic melt host, a mechanism previously not recognized or accepted.

Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr–Nd isotopic, zircon U–Pb dating and Hf isotopic evidences

Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and eNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive eHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and eNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative eHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive eHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive eHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.

Petrogenesis and tectonic significance of the ∼ 850 Ma Gangbian alkaline complex in South China: Evidence from in situ zircon U–Pb dating, Hf–O isotopes and whole-rock geochemistry

The Gangbian alkaline complex in the southeastern Yangtze Block (South China) is composed of Si-undersaturated pyroxene syenites and Si-saturated to -oversaturated syenites and quartz monzonites. SIMS zircon U–Pb analyses indicate that the complex was emplaced at 848 ± 4 Ma, during a previously-recognized interval of magmatic quiescence between the ca 1.0–0.89 Ga Sibaoan orogenic magmatism and the ca 0.83–0.78 Ga magmatic flare-up. The Gangbian rocks are characterized by wide, coherent variations in major and trace elements (SiO 2 = 47.6–68.4%, K 2O + Na 2O = 4.5–10.5%, K 2O/Na 2O = 0.4–1.2, MgO = 1.2–8.5%, Cr = 4.5–239 ppm, and Ni = 4.5–143 ppm) and by enrichment in LIL and LREE and depletion in Nb, Ta and P in trace element spidergrams. Their whole-rock εNd( T) (− 6.5 to − 0.4) and εHf( T) (− 10.7 to 0.4) are positively correlated, suggesting involvement of both metasomatized mantle and continental crust materials in their genesis. In situ zircon Hf–O isotopic measurements for the most evolved quartz monzonite sample yield a binary mixing trend between the mantle- and supracrustal-derived melts. It is suggested that the pyroxene syenites were derived by partial melting of metasomatized, phlogopite-bearing lithospheric mantle, and the parental magma experienced extensive fractionation of pyroxene and olivine associated with varying degrees of crustal contamination. Subsequent fractional crystallization of hornblende and minor amounts of plagioclase from the alkali basaltic magmas, accompanied by crustal contamination, produced the Si-saturated to -oversaturated syenites and quartz monzonites. These ca. 0.85 Ga alkaline rocks and neighboring contemporaneous dolerite dykes are the products of the anorogenic magmatism after the Sibao Orogeny. They post-date the final amalgamation between the Yangtze and Cathaysia Blocks, most likely manifesting the initial rifting of South China within the Rodinia supercontinent.

Geochemistry, tectonics, and crustal evolution of basement rocks in the Eastern Rhodope Massif, Bulgaria

Orthogneisses derived from granitoids with Variscan protolith ages dominate the lower unit of high‐grade metamorphic basement of the Eastern Rhodope Massif in south Bulgaria. We present whole‐rock geochemistry and Sr–Pb isotopic composition of these orthogneisses, which are compared with Pb isotopes of parametamorphic rocks, and hydrothermal ore deposits and associated rocks, to better constrain their composition, origin, and contribution to late Alpine hydrothermal processes. The igneous mineral assemblage is partly preserved, and the field textures and microstructures of the orthogneisses are consistent with a ductile, amphibolite‐grade tectono‐metamorphic overprint during Alpine time, when they were involved in the metamorphic nappe stack. Whole‐rock geochemistry revealed compositions of the orthogneisses largely unaffected by the amphibolite‐grade metamorphism, displaying a magmatic differentiation trend of the igneous protoliths. The protoliths are peraluminous medium‐K calc‐alkaline S‐type granitoids, whose tectono‐magmatic setting discrimination consistently indicates a continental volcanic arc origin. The orthogneisses present trace element and rare‐earth elements (REE) patterns based on which a group of high‐field strength elements‐depleted and REE fractionated orthogneisses and a group of LREE‐enriched orthogneisses can be distinguished. Both geochemical groups show compositions similar to the bulk and upper continental crust and its sedimentary counterparts. Crustal Pb isotope ratios (206Pb/204Pbi = 18.24–18.66) of the orthogneisses are comparable to the paragneisses (206Pb/204Pbi = 18.31–18.93) and uniform in both (207Pb/204Pbi = 15.64–15.72) and 208Pb/204Pbi ratios in the paragneisses (38.23–38.60) and the orthogneisses (38.32-38.56). The trace element data and 87Sr/86Sri isotopes of the orthogneisses (0.7050–0.7117) overlap those of the parametamorphic rocks (0.7039–0.7144), and confirm the supra‐crustal origin of the igneous precursors. A heterogeneous crustal source region is suggested in which melting and crustal contamination during magma genesis with subsequent fractional crystallization was involved in the petrogenesis. Comparative Pb isotope systematics suggests that a significant crustal Pb input to ore‐forming hydrothermal fluids was derived primarily from the metamorphic basement, implying that the brittlly deformed basement during crustal extension acted as an immediate environment for fluid leaching during late Alpine hydrothermal ore‐forming processes.

Zircon U–Pb age and Sr–Nd–Hf isotope geochemistry of the Panzhihua A-type syenitic intrusion in the Emeishan large igneous province, southwest China and implications for growth of juvenile crust

The late Permian Panzhihua syenitic stock in the Pan-Xi area, SW China, showing the typical association of mantle-derived mafic and alkaline rocks along with silicic units, is genetically related to the upwelling Emeishan plume head. This syenitic intrusion consists mainly of metaluminous syenite, metaluminous to peralkaline quartz syenite with subordinate syenite porphyry and alkali-feldspar granite. SHRIMP U–Pb zircon analysis reveal that the Panzhihua syenitic pluton was emplaced at 253.1 ± 1.9 Ma, and is slightly younger than the crystallization age of the spatially associated gabbroic intrusion. These syenitic and granitic rocks have a distinctive A-type chemistry characterized by elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios (3.95 to 6.79). Nb/Ta and Zr/Hf ratios of the syenitic and granitic rocks, gabbros and syenodiorites are similar to those typical of oceanic island basalt (OIB). These granitoids all have relatively low initial 87Sr/ 86Sr ratios (0.7037–0.7058), positive ε Nd( t) values (+ 2.4 to + 3.5). The magmatic zircons from the syenites exhibit positive ε Hf( t) values between 5.9 and 10.4, which correspond to single-stage depleted mantle Hf model ages ( T DM1) of 494 to 671 Ma. In contrast, the magmatic zircons from the syenite porphyries are characterized by positive ε Hf( t) values of 11.4 to 12.9 with younger T DM1 model ages of 382 to 449 Ma than those of the syenites. The positive ε Nd( t) and ε Hf( t) values of the Panzhihua syenitic intrusion indicate a significant contribution of plume-derived basaltic underplating to magma genesis. The similarity of the Nd isotopic compositions of the syenitic and granitic rocks, gabbros and syenodiorites suggests that their parental magmas were derived from a common reservoir. We propose that the Panzhihua syenitic intrusion was predominantly generated by the emplacement of two distinct syenitic melts from differentiation of newly underplated, mildly alkaline basaltic magmas ponded at depth, with incorporation of little Neoproterozoic lower crustal melts, and subsequent fractional crystallization of individual melts. It is therefore demonstrated that the growth of juvenile crust through mantle-derived underplating during the late Permian was significant in the inner zone of the Emeishan large igneous province.

The formation of eucrites: Constraints from metal‐silicate partition coefficients

Abstract— This study explores the controls of oxygen fugacity and temperature on the solubilities of Fe, Ni, Co, Mo, and W in natural eucritic liquids to better constrain the formation of eucritic melts. The solubilities of all five elements in molten silicate in equilibrium with FeNiCo‐, FeMo‐, and FeW‐ alloys increase with increasingly oxidizing conditions and decrease with decreasing temperatures. In applying these data to formation scenarios of the eucrite parent body, we find that the siderophile element abundances in eucrites (meteoritic basalts) cannot be explained by a single‐step partialmelting process from a chondritic, metal‐containing source. The Ni content of the partial melt is too high, and the W and Mo contents are too low compared to the abundances in eucritic meteorites. But Fe, Ni, and Co concentrations in eucrites can be modeled by metal‐silicate equilibrium during more or less complete melting of the eucrite parent body with subsequent fractional crystallization of olivine and orthopyroxene. However, the computed values of Mo are still too low and those of W too high when compared with Mo and W abundances in eucritic meteorites. One possibility is that the Mo and W partition coefficients strongly depend on pressure, although the howardite‐eucrite‐diogenite (HED) parent body only had a minimal pressure gradient (maximum interior pressure = 0.1 GPa). Alternatively, sulfides may have played some role in establishing Mo abundances.

The geochemistry of high-Mg andesite and associated adakitic rocks in the Musoma-Mara Greenstone Belt, northern Tanzania: Possible evidence for Neoarchaean ridge subduction?

Geochemical data are presented for Neoarchaean metavolcanic and plutonic rocks from the Musoma-Mara Greenstone Belt (MMGB) in the northern part of the Tanzania Craton with the aim of inferring their petrogenesis and tectonic settings in which they formed. The MMGB is underlain by two unusual magmatic suites: high magnesium andesites and an adakitic suite. The high-Mg andesites are composed of intermediate volcanic rocks of andesitic composition which, when compared to normal island arc andesites, are characterised by high contents of MgO (2.42–9.47 wt%, Cr (41–797 ppm) and Ni (11–254 ppm). Ratios of Nb/Y are 0.17–0.28, La/Yb are 9.87–22.5 and Sr/Y are 16.1–48.7. These geochemical features are analogous to those shown by modern High Magnesium Andesites (HMA). The adakitic suite is composed of dacites and Na-granitoids and is characterized by Al 2O 3 contents of 14.75–18.54 wt%, variable contents of MgO (0.31–4.96 wt%), Cr and Ni (5.10–250 and 1.22–115 ppm, respectively). Their ratios of Nb/Y are 0.17–0.66, La/Yb are 11–80 and Sr/Y are 20–131. Compared to the HMA, rocks of the adakitic suite are characterized by lower MgO, Cr and Ni contents, higher contents of Al 2O 3 and Sr, higher ratios of La/Yb and Sr/Y and are compositionally similar to modern adakites. Rocks from both suites show negative anomalies of Nb, Ta and Ti and have similar ɛNd values (at 2.67 Ga) of +0.44 to +2.66. The geochemical characteristics of the HMA are consistent with the derivation of their parent magma by partial melting of mantle peridotite that has been fluxed by slab-derived aqueous fluids above a continental arc. As the slab further descended into the mantle, partial melting of the subducted oceanic crust occurred in the garnet stability field producing a melt that was depleted in HREE. The slab-derived melts percolated into the mantle wedge and reacted with mantle peridotite resulting in parental magmas of rocks of the adakitic suite. Subsequently, the parental magmas of both rock suites ascended through and were contaminated by older felsic crust forming the continental arc basement. Subsequent fractional crystallization of pyroxene and hornblende led to the range in Mg numbers, CaO, Cr and Ni contents observed in the rocks. The association of some members of the adakitic suite with locally derived clastic sedimentary rocks suggests that the latest volcanic episode in the MMGB occurred in a continental back arc basin. The rapid emplacement of volcanic and plutonic rocks in a relatively short time interval is best explained in terms of the ridge-subduction model of [Iwamori, H., 2000. Thermal effects of ridge subduction and its implication for the origin of granitic batholith and paired metamorphic belts. Earth Planet. Sci. Lett., 181, 131–144.] whereby subduction of the ridge crest results in anomalous high thermal input into the subduction zone leading to rapid arc magmatism within a few tens of kilometers from the slab–crust interface and within a time interval of 30 Ma after ridge subduction.

Magmatic evolution of the Puyehue–Cordón Caulle Volcanic Complex (40° S), Southern Andean Volcanic Zone: From shield to unusual rhyolitic fissure volcanism

Magmas erupted from Quaternary volcanoes of Southern Andes between 37° and 46° S latitude are mainly basaltic to andesitic. However, PCCVC (40° S) shows a singular magmatic evolution due to the abnormal evacuation of rhyolites, especially in the last 100 ka. In addition, PCCVC is the result of juxtaposing products from the NW-trending alignment of Cordillera Nevada caldera, Cordón Caulle fissure volcano and the Puyehue stratocone. Using 40Ar/ 39Ar and 14C geochronology it can be established that they evolved since ca. 500 ka as coeval but separated vents with a first stage of shield volcanism, followed by repeated collapses that formed an internal NW-elongated graben. From ca. 100 ka, volcanic activity occurred in both a fissure system (Cordón Caulle) and a central volcano (Puyehue). Holocene explosive eruptions, mainly in the Puyehue crater, accompanied the dome growing along a NW-trending fissure system. Last historical eruptions were in 1921 and 1960 when NW fissures of Cordón Caulle fed rhyodacitic lava flows. In 1960, the fissure eruption was triggered by a remote Mw: 9.5 thrust earthquake. Cordillera Nevada caldera presents a reduced compositional range (52–63% SiO 2) and geochemical features of low-pressure magma mixing and assimilation. Instead, Cordón Caulle and Puyehue volcanoes have a wide silica range (48–71% SiO 2) and an outstanding affinity, which can be modelled with initial high-pressure fractional crystallization, moderate magma mixing and subsequent low-pressure fractional crystallization from a common parental source. The exceptional magmatic evolution and eruptive style of PCCVC in Southern Andes could be related with the physics of the plumbing system, which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime.

Mantle Xenoliths and Their Host Magmas in the Eastern Alkaline Province, Northeast Mexico

Alkaline magmas along the periphery of Sierra de San Carlos-Cruillas and Sierra de Tamaulipas (Eastern Alkaline Province, northeast Mexico) contain ultramafic xenoliths. Most of the host rocks are mafic (e.g., basalts, basanites, trachybasalts, phonotephrites), and have geochemical characteristics of nearly primary magmas generated in subcontinental zones (SiO2 = 42.6-48.4%, MgO = 6.3-11.2%, Mg# = 59.2-69.7). MORB-normalized patterns are similar to those displayed by extensionrelated mafic magmas. This hypothesis is supported by an enrichment in light REE ([La/Yb]N = 10.8-27.1; chondrite-normalized ratios) and the behavior of relatively immobile trace elements. Using a partial melting model for REE, the source for the mafic magmas apparently was in the garnet P-T stability field. On the other hand, peralkaline phonolites and tephriphonolites (SiO2 = 52.9-54.1%, MgO = 1.5-1.8%, Mg# = 33.1-39.8) also contain mantle xenoliths. These evolved magmas show MORB-normalized multi-element diagrams characterized by enrichment in highly incompatible elements (e.g., Rb, Sr, Ba) and positive anomalies for HFSE. Ultramafic xenoliths are predominantly protogranular spinel-lherzolites as well as harzburgites and rare dunites, websterites, and wherlites. A few samples display transitional and porphyroclastic textures, indicating that the xenoliths were carried to the surface from stable mantle zones with little or no deformation. The mineralogy (olivine + orthopyroxene + clinopyroxene ± spinel) of xenoliths contained in mafic magmas is typical of unaltered mantle nodules. However, small garnet crystals exhibiting disequilibrium textures occur in one sample. The core and rim compositions in olivine range from Fo90 to Fo94, whereas orthopyroxenes are characterized by En88-93 and clinopyroxenes by En44-51Fs3-10Wo43-50. Chromiferous spinels have Mg/(Mg + Fe+2) = 0.76-0.83 and Cr/(Cr + Al) = 0.10-0.25. In comparison to the nodules contained in mafic magmas, mantle xenoliths included in phonolitic liquids show some differences: (1) smaller size (diameter < 1 cm); (2) clinopyroxene is less common (< 5% volume) and spinel is absent; and (3) partial alteration to micaceous minerals. Equilibrium temperatures for mantle xenoliths contained in mafic magmas range from 850 to 1170°C, as calculated applying different geothermometers, whereas mantle nodules sampled by peralkaline liquids show only lower equilibrium temperatures (<900°C). A first approximation to equilibrium pressure, based on mineralogical constrains, indicates values from 10 to 25 kbar. Summing up, we consider that the mafic magmas were generated within the spinel-garnet domain of the lithospheric mantle, having little or no interaction with their wall rocks. The ultramafic xenoliths were probably sampled during magma ascent above its source. In contrast, the geochemistry of phonolitic rocks and their altered mantle xenoliths reveal that such magmas cannot be explained by direct mantle melting. The petrogenesis of these peralkaline magmas can be described by a two-step model: (1) partial melting of metasomatized mantle which produced an alkaline magma enriched in LILE and HFSE; and (2) subsequent fractional crystallization of this magma at upper mantle pressures, producing phonolites. While ascending, the peralkaline magmas sampled altered nodules in a shallow level of the subcontinental mantle. The rise and eruption of the mantle-bearing mafic and evolved magmas were facilitated by the post-Laramide extensional regime established during Tertiary time in northeast Mexico.

Geochemistry and Sr, Nd, Pb isotopic composition of the Central Atlantic Magmatic Province (CAMP) in Guyana and Guinea

The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, extending more than 5000 km north to south, on both sides of the Atlantic Ocean. Its emplacement occurred about 200 Ma ago, at the Triassic–Jurassic boundary, and is linked to the initial breakup of Pangaea. Two areas of the province are studied here: French Guyana/Surinam (South America) and Guinea (West Africa), in order to document the petrogenesis and geodynamical significance of high-Ti and low-Ti basaltic magmas from the CAMP. In Guyana, doleritic and gabbroic dykes are located on the edge of the Guiana Shield, and represent limited volumes of magma. They display low SiO 2 (47–50%), high TiO 2 (2.5–3.5%) and high FeO tholeiitic trends and show variably enriched trace element patterns ((La/Yb) n=1.5–5.1). Their isotopic signature and ratios of very incompatible elements (εNd i=+5.8 to +4.2, ( 87Sr/ 86Sr) i=0.703–0.705, ( 207Pb/ 204Pb) i=15.46–15.64) match a depleted PREMA (prevalent mantle)-like source. Their genesis can be modeled by ca. 15% partial melting of a lherzolite source, and a subsequent limited fractional crystallization (5–10%) or a slight upper crustal assimilation–fractional crystallization (AFC, r=0.1, Proterozoic contaminant). In Guinea, in contrast, huge volumes of CAMP magmas were intruded along the Rockelides suture and the West African craton, forming the Fouta Djalon sills and the Kakoulima laccolith. The laccolith is more than 1000 m thick. These features consist of gabbros, dolerites, diorites and mafic (gabbro) and ultramafic (dunite, wherlite) cumulates. Guinean tholeiites show high SiO 2 (51–58%), low TiO 2 (0.7–1.2%) and FeO trends, with high LILE/HFSE ratios and slight negative Nb–Ta anomalies. Isotopic signatures (εNd i=+0.4 to −5.3, ( 87Sr/ 86Sr) i=0.705–0.710, ( 207Pb/ 204Pb) i=15.57–15.66) indicate a more enriched source than for Guyana as well as a higher rate of magma–upper crust interaction through an AFC process ( r=0.3, Birimian crust contaminant) and, probably, an additional upper crustal contamination for the most differentiated sample. This geochemical study supports the prevalence in Guinea, as for other low-Ti CAMP tholeiites, of a lithospheric mantle source, previously enriched during ancient subduction events, and preferentially reactivated in late Triassic times by edge-driven convection between cratonic and mobile belt domains. A larger contribution from a depleted asthenospheric source is required to generate high-Ti tholeiites in Guyana, which may reflect the development of CAMP rifting towards the initiation of the Central Atlantic oceanic crust.

Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite

Secondary ion mass spectrometer (SIMS) oxygen isotope analyses were performed on 24 clasts, representing 9 clast types, in the Dar al Gani (DaG) 319 polymict ureilite with precisions better than 1‰. Olivine-rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Other igneous clasts, including plagioclase-bearing clasts, also plot along the CCAM line, indicating that they were derived from the ureilite parent body (UPB). Thus, we suggest that some of the plagioclase-bearing clasts in the polymict ureilites represent the “missing basaltic component” produced by partial melting on the UPB. Trace element concentrations (Mg, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr and Ba) in ureilitic plagioclase and glass from 13 clasts were obtained by using the SIMS high mass resolution method. The trace element contents of the plagioclase generally show monotonic variations with anorthite content (mol%) that are consistent with partial melting and fractional crystallization. Incompatible trace element concentrations (K, Ti, and Ba) are low and variable for plagioclase with An > 40, indicating that the parental magmas for some clasts were derived from a depleted source. We performed partial melt modeling for CI and CM precursor compositions and compared the results to the observed trace element (K, Ba, and Sr) abundances in the plagioclase. Our results indicate that (1) the UPB evolved from a alkali-rich carbonaceous chondritic precursor, (2) parent melts of porphyritic clasts could have formed by 5–20% equilibrium partial melting and subsequent fractional crystallization, and (3) parent melts of the incompatible trace element-depleted clasts could be derived from fractional melting, where low degree (<10%) partial melts were repeatedly extracted from their solid sources. Thus, both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in DaG-319 suggest that the UPB underwent localized low degree-partial melting events. The partial melts could have been repeatedly extracted from the precursor, resulting in the formation of the olivine-pigeonite monomict ureilites as the final residue.

An adakitic pluton on Kyushu Island, southwest Japan arc

An adakitic pluton is designated the Shiraishino granodiorite ca. 15 km2 in outcrop area on Kyushu Island, southwestern Japanese arc. Sr, Y, and rare earth element concentrations and crystallization characteristics of the pluton are similar to those of adakites. Geochemical and tectonic considerations show that the pluton was derived from partial melting of a subducted slab with minor contamination of high 87Sr/86Sr material and subsequent fractional crystallization of plagioclase+hornblende.Compiled Sr and Y data indicate that most Cretaceous granitic rocks of southwestern Japan, generated by partial melting of mafic lower crust. Adakitic plutons derived by slab melting occur rarely and have older ages. It is concluded that plutonism in the southwestern Japanese arc during Cretaceous time was began with the emplacement of small adakitic intrusions and progressed to more abandant granitic rocks of the type derived from the mafic lower crust.

Geochemistry of lower crustal xenoliths from Neogene Hannuoba basalt, North China craton: implications for petrogenesis and lower crustal composition

Thirty granulite and pyroxenite xenoliths from the Neogene Hannuoba basalt of the North China craton have been analyzed for major and trace element compositions. The granulites range in composition from mafic to felsic with SiO2 = 45.7 to 73.0% and also contain metasediments. The compositions of mafic and intermediate granulites can be explained by fractional crystallization of a magma chamber in the lower crust. The magmatic granulite xenoliths are interpreted as product of basaltic underplating and subsequent fractional crystallization at the base of the crust. Thermobarometric studies and correlation of calculated P-wave velocities with regional seismic refraction results suggest that the upper part of the lower crust, which accounts for two thirds of the entire lower crust in the North China craton at a 24- to 38-km depth, is dominated by intermediate and felsic compositions. Only the lowermost crust (38–42 km) has a mafic composition. This is also supported by the abundance of intermediate and felsic granulite xenoliths, which account for 45% of the granulite population collected. The calculated bulk lower crust in the Hannuoba area has an intermediate composition with SiO2 = 58%. Because other parts of North China also show a similar velocity structure, with the high-velocity layer confined to the lowermost 3 to 5 km of crust, the results from the Hannuoba area are considered to be representative of the reactivated North China craton as a whole.

(-)-(1R,2S,2'R,5R)-2-(1-hydroxyprop-2-yl)-5-methylcyclohexanol.

Stereoselective hydroboration of (−)-isopulegol and subsequent fractional crystallization furnishes the title compound, C10H20O2. The relative configuration of the stereogenic centres has been assigned by means of X-ray diffraction analysis since the monoterpenediol is employed as a versatile chiral building block in stereospecific natural product synthesis.