查看更多>>摘要：As an early crystallizing phase in the magma, apatite can accommodate a wide range of multivalent cations and has a capacity resistant to weathering and hydrothermal alteration. These features enable apatite to record the primary redox state of precursor magma. Here, we present new electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) geochemical data on apatite and zircon grains from the epithermal gold ore-hosting Dahalajunshan Formation volcanics in the Tulasu basin (Western Tianshan Orogen, Central Asia). Calculated zircon Ce-/Ti-based oxybarometric results and sulfur content of apatite suggest that the parental magma of the volcanics was relatively oxidized. Furthermore, we compiled a regional apatite chemical dataset to assess whether other elements (Mn, V, Eu, Ce, As and Ga) in apatite can serve as a magma redox proxy. Negative correlation between apatite Mn and whole-rock MgO is evident, implying the control of magma fractionation on apatite Mn contents. Also, oxygen fugacity (fO2) calculated by Mn-in-apatite oxybarometer correlates strongly with magma fractionation, which indicates that fractional crystallization (instead of fO2) controlled the Mn-uptake in apatite. The apatite As budget is sensitive to_fO2 variation but does not show any correlation with the ratio of nonbridging oxygens to tetrahedral cations (NBO/T), whole-rock SiO2, or aluminum saturation index. In contrast, apatite V and Ga contents correlate obviously with whole-rock SiO2 and NBO/T but not fO2, and apatite Eu and Ce anomalies show no correlation with fO2. Therefore, we conclude that only the apatite As and S contents can reflect the magma redox state. The calculated fO2 of the Dahalajunshan Formation volcanic rocks is AFMQ+0.53 ± 0.52 (based on the fayalite+magnetite+quartz buffer), which is markedly lower than those of typical porphyry Cu ore-forming magmas worldwide. Thus, from the perspective of magma fO2, regional magma is inferred to have little potential to form the porphyry Cu deposit.
查看更多>>摘要：Development of computational modeling tools has revolutionized studies of magmatic processes over the last four decades. Their refinement from binary mixing equations to tiiermodynamically controlled geochemical assimilation models has provided more comprehensive and detailed modeling constraints of an array of magmatic systems. One of the questions that has not yet been vigorously studied using thermodynamic constraints is die origin of massif-type anordiosites. The parental melts to these intrusions are hypodiesized to be either mantle-derived high-Al basaltic melts that undergo crustal contamination or monzodioritic melts derived directly from lower crust. On the otiier hand, many studies suggest tiiat die monzodioritic rocks do not represent parental melts but instead represent crystal remnants of residual liquids left after crystal fractionation of parental melts. Regardless of the source or composition, magmas tliat produce massif-type anordiosites have been suggested to have undergone polybaric (—1000-100 MPa) fractional crystallization while ascending through die lithosphere. We conducted lower crustal melting, assimilation-fractional crystallization, and isobaric and polybaric fractional crystallization major element modeling using two thermodynamically constrained modeling tools, die Magma Chamber Simulator (MCS) and rhyolite-MELTS, to test the suitability of tiiese tools and to study the petrogenesis of massif-type anordiosites. Comparison of our models with a large suite of whole-rock data suggests that the massif-type anorthosite parental melts were high-Al basalts that were produced when hot mantle-derived partial melts assimilated lower crustal material at Moho levels. These contaminated basaltic parental magmas tiien experienced polybaric fractional crystallization at different crustal levels (—40 to 5 km) producing residual melts tiiat crystallized as monzodioritic rocks. Model outcomes also support die suggestion that the cumulates produced during polybaric fractional crystallization likely underwent density separation, thus producing die plagioclase-rich anortiiositic rocks. The modeled processes are linked to a four-stage model that describes the key petrogenetic processes that generate massif-type anorthosites. The presented framework enables further detailed thermodynamic and geochemical modeling of individual anorthosite intrusions using MCS and involving trace element and isotope constrains.
查看更多>>摘要：High-silica volcanic rocks and their plutonic counterparts carry critical information concerning the evolution of shallow magmatic systems and construction of silicic upper continental crust, yet their origins remain debatable. Here, we examine high-silica volcanic rocks from the Lower Cretaceous Manitu and Baiyin'gaolao formations of the Wuchagou volcanic field, central Great Xing'an Range (GXR), to constrain the genesis of these high-silica volcanic rocks and the evolution of the Wuchagou magmatic system. Geochronological data suggest that the Manitu and Baiyin'gaolao volcanic rocks represent a prolonged eruption from the Wuchagou magmatic system, spanning Period I (135-129 Ma) and Period II (127-122 Ma) magmatism in the central GXR. Their slightly depleted ε_(Nd)(t) values and relatively young model ages coincide with a juvenile crustal origin. We suggest that the Manitu and Baiyin'gaolao high-silica volcanic rocks are the evolved compositions of their coexisting intermediate rocks given the similarities in both formation age and Sr-Nd isotopic composition and systematic geochemical variations. Clinopyroxene-and amphibole-based thermobarometers respectively yield low average pressures of —3.3 kbar and — 1.8 kbar with low temperatures of —1013 °C and 875 °C. Also, Clinopyroxene-and amphibole-based hygrometry yield relatively high H2O contents of —2.7-4.1 wt% and — 3.1-4.6 wt%. Together, they indicate the presence of a hydrous upper-crustal magma reservoir feeding the eruptions of the Manitu and Baiyin'gaolao high-silica volcanic rocks. Combining the behavior of highly incompatible elements (Rb and Th) with trace element modeling results, we show that the Wuchagou high-silica volcanic rocks formed by protracted extraction of high-silica melts in the upper crust, with complementary residual cumulates partially disguised in the Early Cretaceous felsic plutons in the central GXR. Our results highlight the significance of prolonged upper-crustal differentiation of magmas derived from juvenile crust in the development of the high-silica upper continental crust.
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