depleted compositions occur in the upper part of the lithospheric Geological and geophysical evidence indicates that at least 100 km mantle, which now is >100 km thick. Garnet peridotites are of Archaean to Proterozoic lithospheric mantle has been removed essentially undepleted, and Y–Ga–Zr relationships of the garnets from beneath large areas of eastern and southeastern China during are typical of Phanerozoic mantle. The overall highly fertile nature late Mesozoic to Cenozoic time. Mantle-derived xenoliths in Tertiary of the existing lithosphere requires that the Archaean and Proterozoic basalts from several localities across this region have been studied mantle that existed beneath the region in Palaeozoic times has been by X-ray fluorescence, electron microprobe and laser ablation microlargely or completely removed, and replaced by younger, hotter and probe–inductively coupled plasma-mass spectrometry to characterize more fertile material. This probably occurred by upwelling of this thinner lithosphere. Trace element patterns of clinopyroxenes in asthenospheric material during late Mesozoic to Cenozoic time, the peridotites from southeastern China can be divided into four underplating to form new lithosphere. The occurrence of rare depleted groups: fertile garnet lherzolites, fertile spinel (± garnet) lherzolites, xenoliths may show that some older mantle material is residual and and depleted and enriched peridotites. The addition of Nb, Sr, light coexists with younger material beneath southeastern China. rare earth elements, but not of Ti and Zr, suggests a metasomatizing agent containing both H2O and CO2. This study also demonstrates that the negative Ti anomaly commonly observed in clinopyroxene from mantle peridotites cannot be balanced by the Ti in coexisting orthopyroxene, but can be explained by small degrees of partial
Abstract Collision‐related porphyry Cu deposits (PCDs) are restricted to previous magmatic arcs, in which sulfide‐rich lower crust occurred. Fertile adakite‐like porphyries associated with PCDs have higher K 2 O contents and K 2 O/Na 2 O ratios than barren porphyries emplaced in the same arc. The elevated K 2 O/Na 2 O ratios of fertile porphyries reflect substantial inputs of coeval hydrous, oxidized ultrapotassic melts. Input of such melts could increase the water content and oxygen fugacity of the lower‐crust‐derived melts, which in turn would promote the breakdown of sulphides in the lower crust and increase the contents of Cu and S in the melts, making them favourable for the formation of large PCDs. The relatively low K 2 O contents and K 2 O/Na 2 O ratios of barren porphyries indicate limited input of ultrapotassic melts; these magmas have low potential to form PCDs. Thus, the input of ultrapotassic melts into the sulfide‐rich juvenile lower crust drives the formation of collision‐related PCDs.
Most of Earth’s volcanism occurs at tectonic plate boundaries associated with subduction or rifting processes. The mantle plume hypothesis is an important supplement to plate tectonics for explaining some high-volume intraplate volcanic fields. However, many intraplate magmatic provinces occur as low-volume, monogenetic basaltic-suite fields that are neither associated with plate-boundary processes nor attributable to mantle plumes, and the origin of such magmatism has long been debated. Identification of their source characteristics and possible mechanisms that trigger mantle melting will provide essential insights into Earth’s mantle heterogeneity and also develop our knowledge of tectonic plate movement through time. Here, we report new geochronology, mineral chemistry (especially olivine), and whole-rock chemical and Sr-Nd-Pb-Hf isotopic compositions on Cenozoic intracontinental alkaline basalts from the northwestern Tarim craton (central Asia), aiming to better assess the origin of Earth’s low-volume effusive intraplate volcanic fields. The basalts (ca. 42 Ma) have olivine (e.g., mean Ni abundances of ∼2250 ppm, mean Mn/Zn ratios of 13.7) and whole-rock chemistry consistent with their derivation from a mixed peridotite-pyroxenite source. Moderately depleted Sr-Nd-Pb-Hf isotopes (87Sr/86Sr = 0.7039−0.7053; εNd = +4.0 to +5.5; 206Pb/204Pb = 18.247−18.535; εHf = +8.1 to +8.7) require a young (ca. 500 Ma) oceanic crust recycled into the source, possibly related to subduction events during the assembly of Pangea. Estimated thermal-chemical conditions indicate that the original melting occurred in a relatively dry (H2O = 1.4 ± 0.9 wt%) and reduced (logfO2 ΔFMQ = −0.97 ± 0.21, where FMQ is fayalite-magnetite-quartz) asthenosphere under a mantle potential temperature of ∼1420 °C and a pressure of ∼3.7 GPa (corresponding to a depth of ∼120 km). Combining these data with regional tectonic history and geophysical data (high-resolution P-wave tomography), we propose that the long-lasting India-Eurasia collision triggered asthenospheric upwelling, focusing melts along translithospheric zones of weakness; this model provides a robust explanation for the observed Cenozoic intracontinental volcanism in central Asia. The integrated geochemical and geophysical evidence reveals that plate subduction−induced mantle upwelling represents a likely mechanism for the generation of many regions of plume-absent intraplate magmatism within continents.
A detailed in situ isotopic (U–Pb, Lu–Hf) and geochemical study of zircon populations in a composite sequence of foliated to massive Cambro-Ordovician intrusions in the Deep Freeze Range (North Victoria Land, Antarctica), has highlighted great complexity in zircon systematics. Zircons in deformed granitoids and tonalites display complex internal textures, a wide spread of concordant U–Pb ages (between 522 and 435 Ma) and unusual trace-element compositions (anomalous enrichment of light rare earth elements, U, Th and Y) within single zircon grains. In contrast, zircons from undeformed samples display a limited range of U–Pb ages and trace-element compositions. Zircons from all age and textural populations in most of the deformed and undeformed samples show a relatively narrow range of εHf values, suggesting that the Lu–Hf system remained undisturbed. Inferred emplacement ages cover a time interval of about 30 Myr: from 508 to 493 Ma for the oldest strongly foliated synkinematic Howard Peaks megacrystic monzogranites and high-K calc-alkaline mafic to intermediate rocks of the 'Corner Tonalite' unit; from about 489 to 481 Ma for the younger massive shoshonitic mafic dyke suite and the high-K calc-alkaline Keinath granite. The observed isotopic and chemical variations in zircon are attributed to a sub-solidus recrystallization under hydrous conditions and varying temperature, in a setting characterized by a transpressional to extensional stress regime.
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