Abstract The Kerguelen large igneous province (LIP) has been related to mantle plume activity since at least 120 Ma. There are some older (147–130 Ma) magmatic provinces on circum-eastern Gondwana, but the relationship between these provinces and the Kerguelen mantle plume remains controversial. Here we present petrological, geochronological, geochemical, and Sr–Nd–Hf–Pb–Os isotopic data for high-Ti mafic rocks from two localities (Cuona and Jiangzi) in the eastern Tethyan Himalaya igneous province (147–130 Ma). Zircon grains from these two localities yielded concordant weighted mean 206Pb/238U ages of 137.25 ± 0.98 Ma and 131.28 ± 0.78 Ma (2σ), respectively. The analyzed mafic rocks are enriched in high field strength elements and have positive Nb–Ta anomalies relative to Th and La, which have ocean island basalt-like characteristics. The Cuona basalts were generated by low degrees of melting (3–5 %) of garnet lherzolites (3–5 vol% garnet), and elsewhere the Jiangzi diabases were formed by relatively lower degrees of melting (1–3%) of garnet lherzolite (1–5 vol% garnet). The highly radiogenic Os and Pb isotopic compositions of the Jiangzi diabases were produced by crustal contamination, but the Cuona basalts experienced the least crustal contamination given their relatively low γOs(t), 206Pb/204Pbi, 207Pb/204Pbi, and 208Pb/204Pbi values. Major and trace element geochemical and Sr–Nd–Hf–Pb–Os isotope data for the Cuona basalts are similar to those for products of the Kerguelen mantle plume head. Together with high mantle potential temperatures (>1500 °C), this suggests that the eastern Tethyan Himalaya igneous province (147–130 Ma) was an early magmatic product of the Kerguelen plume. A mantle plume initiation model can explain the temporal and spatial evolution of the Kerguelen LIP, and pre-continental breakup played a role in the breakup of eastern Gondwana, given the >10 Myr between initial mantle plume activity (147–130 Ma) and continental breakup (132–130 Ma). Like studies of Re–Os isotopes in other LIPs, the increasing amount of crustal assimilation with distance from the plume stem can explain the variations in radiogenic Os.
Abstract The application of whole-rock compositions to trace magma evolution or crystal-melt equilibrium may be called into question when foreign crystals are incorporated into host magmas. To address this challenge, establishing the origins (orthocrysts, antecrysts, xenocrysts, etc.) of minerals with variable textures in magmatic plumbing systems is necessary. In this paper, we describe complex oscillatory zoning patterns of clinopyroxenes (Cpx) from the Miocene Majuangou (MJG) alkali basalts from West Qinling, China. Our detailed petrographic, mineralogical, and geochemical analyses unravel the origins of various Cpx crystals with distinct textural patterns, thereby providing information about magma storage, recharging and/or mixing, and transportation, as well as the reactions between crystals and melts/fluids. Based on textural patterns, Cpx may be divided into four types: normal (Type-1 Cpx), simple oscillatory (Type-2 Cpx), complex oscillatory (Type-3 Cpx), and grains that lack zoning (Type-4 Cpx, suggested to be orthocrysts). Through the textural characterization of Cpx, the comparison between different types of Cpx, and the relationships between Cpx major compositions from different lithologies, we concluded that Type-1–3 Cpx cores are antecrysts or xenocrysts with diverse origins: primitive magma (Type-1 Cpx cores), magma mush (Type-2 Cpx cores), and crustal granulite (Type-3 Cpx cores). The zoning patterns and the compositions of these Cpx crystals indicate at least three batches of magmatic activity, i.e., the Batch-1 low-Mg# magma (Mg#: 47.4–53.3), the Batch-2 primitive magma (Mg#: 57.2–64.5), and the Batch-3 low-Mg# host alkali magma (Mg#: 47.2–54.6). Cpx-melt thermobarometry demonstrates that at least two crustal magma reservoirs existed in the magma plumbing system at depths of 30.1 and 40.9 km. The antecrystic/xenocrystic Cpx cores were captured by, continued to grow in, and subsequently reacted with ascending K-rich melt/fluid. The spongy textures in Cpx cores/mantles are attributed to this reaction, which may be expressed as: Melt 1 (primitive or evolved) + K-rich melts/fluids + Cpx (CaMgSi2O6) = K-feldspar (KAlSi3O8) + ilmenite (FeTiO3) + Melt 2 (derivative). The products of this reaction (K-feldspar and ilmenite) filled the sieves in the spongy zones of Type-1–3 Cpx. This detailed investigation of compositional and textural features of Cpx antecrysts/xenocrysts suggests that the interactions between various interconnected magma reservoirs are widespread beneath the magmatic plumbing system. Our study emphasizes the importance of the incorporation of foreign crystals and the Cpx-melt/fluid reaction in magmatic plumbing system, which can significantly modify the whole-rock compositions and lead to the formation of spongy textures without the need for fractures and cracks in minerals.
Porphyry Cu (–Mo–Au) deposits occur not only in continental margin–arc settings (subduction-related porphyry Cu deposits, such as those along the eastern Pacific Rim (EPRIM)), but also in continent–continent collisional orogenic belts (collision-related porphyry Cu deposits, such as those in southern Tibet). These Cu-mineralized porphyries, which develop in contrasting tectonic settings, are characterized by some different trace element (e.g., Th, and Y) concentrations and their ratios (e.g., Sr/Y, and La/Yb), suggesting that their source magmas probably developed by different processes. Subduction-related porphyry Cu mineralization on the EPRIM is associated with intermediate to felsic calc-alkaline magmas derived from primitive basaltic magmas that pooled beneath the lower crust and underwent melting, assimilation, storage, and homogenization (MASH), whereas K-enriched collision-related porphyry Cu mineralization was associated with underplating of subduction-modified basaltic materials beneath the lower crust (with subsequent transformation into amphibolites and eclogite amphibolites), and resulted from partial melting of the newly formed thickened lower crust. These different processes led to the collision-related porphyry Cu deposits associated with adakitic magmas enriched by the addition of melts, and the subduction-related porphyry Cu deposits associated with magmas comprising all compositions between normal arc rocks and adakitic rocks, all of which were associated with fluid-dominated enrichment process. In subduction-related Cu porphyry magmas, the oxidation state (fO2), the concentrations of chalcophile metals, and other volatiles (e.g., S and Cl), and the abundance of water were directly controlled by the composition of the primary arc basaltic magma. In contrast, the high Cu concentrations and fO2 values of collision-related Cu porphyry magmas were indirectly derived from subduction modified magmas, and the large amount of water and other volatiles in these magmas were controlled in part by partial melting of amphibolite derived from arc basalts that were underplated beneath the lower crust, and in part by the contribution from the rising potassic and ultrapotassic magmas. Both subduction- and collision-related porphyries are enriched in potassium, and were associated with crustal thickening. Their high K2O contents were primarily as a result of the inheritance of enriched mantle components and/or mixing with contemporaneous ultrapotassic magmas.
Here we report geochronological, geochemical, and Sr-Nd-Hf isotopic data of a basic intrusion exposed in the Jiacha area within the Tethyan Himalaya, southern Tibet. The Jiacha dikes are composed mainly of mafic rocks (gabbros). In zircon U-Pb dating of Jiacha dikes samples, the weighted mean ages of magma emplacement were determined to be 241.5±4.2 to 245.5±3.3 Ma, showing a Middle Triassic magmatic event in the eastern part of the Tethyan Himalaya. The Jiacha dikes have relatively low MgO and total alkali (K2O+Na2O) contents but high TiO2 contents, exhibit weakly fractionated rare earth element (REE) patterns with slight depletions in light REEs and no obvious Eu anomalies, and show enrichment in high field strength elements and depletion in large-ion lithophile elements. Their initial 87Sr/86Sr ratios vary in the range 0.704250–0.704321, with a positive εNd(t = 243 Ma) of +3.07 to +3.67, and their initial 176Hf/177Hf ratios vary in the range 0.282872–0.283044, with a positive εHf(t = 243 Ma) of +8.53 to +14.45. We propose that the Jiacha dikes were derived from spinel lherzolites in the mantle with no crustal contamination and underwent fractional crystallization of clinopyroxene. Considering previous studies, we propose that Jiacha dikes represent oceanic crustal products of the early evolution of the Neo-Tethys, indicating that the ocean had already begun to open in the Middle Triassic.
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