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Petrology and geochemistry of South Mid-Atlantic Ridge(19°S) lava flows:Implications for magmatic processes and possible plume-ridge interactions

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Geoscience Frontiers 2020年第6期11卷 1953-1973页

作者： Haitao Zhang Xuefa Shi Chuanshun Li Quanshu Yan Yaomin Yang Zhiwei Zhu Hui Zhang Sai Wang Yili Guan Renjie Zhao Key Laboratory of Marine Sedimentology and Environmental Geology First Institute of OceanographyMinistry of Natural and ResourcesQingdaoShandong266061China Laboratory for Marine Geology Qingdao National Laboratory for Marine Science and TechnologyQingdaoShandong266061China College of Earth Science and Engineering Shandong University of Science and TechnologyQingdao266590China National Deep Sea Center Ministry of Natural and ResourcesQingdao266061China

The South Mid-Atlantic Ridge(SMAR)19°S segment,approximately located along the line of Saint Helena volcanic chain(created by Saint Helena mantle plume),is an ideal place to investigate the issue whether the ridgehotpot interaction process affected the whole *** this study,we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19°S *** oxides compositions show that all samples are *** contents of compatible trace elements(e.g.,Ni=239-594 ppm and Cr=456-1010 ppm)and low Fe/Mn(54-67)and Ce/Yb(0.65-1.5)ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle *** software PRIMELT3,this study obtained mantle potential temperatures(Tp)beneath the segment of1321-1348℃,which is lower relative to those ridges influenced by mantle *** asthenospheric mantle beneath the SMAR 19°S segment starts melting at a depth of^63 km and ceases melting at^43 km with a final melting temperature of^1265℃.The extent of partial melting is up to 16%-17.6%with an average adiabatic decompression value of 2.6%/*** correlations of major oxides(CaO/Al2 O3)and trace elements(Cr,Co,V)with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface.87Sr/86Sr(0.702398-0.702996),143 Nd/144 Nd(0.513017-0.513177)and 206Pb/204Pb(18.444-19.477)ratios of these lavas indicate the mantle source beneath the SMAR 19°S segment is composed of a three-component mixture of depleted MORB mantle,PREMA mantle,and HIMU mantle *** simple,binary mixing results among components from plume-free SMAR MORB,Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19°S segment may be polluted by both Saint Helena and Tristan plume enriched *** abovementioned mantle potential temperatures,together with the low Sai

关键词： magmatic process Mantle potential temperature Crystallization pressure Plume-ridge interaction South Mid-Atlantic Ridge

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On the Enigma of Nb-Ta and Zr-Hf Fractionation——A Critical Review

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Journal of Earth Science 2011年第1期22卷 52-66页

作者：黄慧牛耀龄赵志丹黑慧欣朱弟成 School of the Earth Sciences and Resources China University of Geosciences Key Laboratory of Western China’s Environmental System (Ministry of Education) School of Earth Sciences Lanzhou University Department of Earth Sciences Durham University

Elemental ratios Zr/Hf and Nb/Ta are expected to be constant and of chondritic value （-36.30 and -17.57, respectively） in mantle and mantle-derived rocks. Studies in recent years have shown, however, that these two ratios do vary in some of these rocks. For example, MORB-like seamount lavas from flanks of the East Pacific Rise （EPR） show a correlated Zr/Hf （-25-50） and Nb/Ta （-9-18） variation. These two ratios are also correlated with ratios of more incompatible over less incompatible elements （e.g., La/Sm, Rb/Cs, Th/U, Nb/U, Sm/Yb） and with radiogenic isotope ratios （e.g., 87 Sr/86Sr, 143Nd/144Nd）. Furthermore, abyssal peridotites, which are melting residues for MORB, also show a huge correlated variation between Zr/Hf （-2.5-335） and Nb/Ta （-1-170）. All these observations plus a correlated variation between Zr/Hf （-22-48） and Nb/Ta （-10-23） in lunar rocks are consistent with the Zr-Hf and Nb-Ta fractionation being of magmatic origin. This contrasts with the common view that geochemical processes cannot readily fractionate them. As charges and ionic radii are the principal factors in the general theory of elemental fractionation, this theory cannot explain the fractionation of these two element pairs with the same charges （i.e., 5＋ for Nb and Ta, and 4＋ for Zr and Hf） and essentially the same ionic size （i.e., RNb/RTa=1.000, Rzr/RHf=1.006 to -1.026 for coordination numbers of 6, 7, 8 and 12）. We explore the possibilities of other factors and processes （e.g., mass-dependent fractionation during magmatism） that may cause the observed Nb-Ta and Zr-Hf fractionation. We emphasize that understanding the correlated Nb-Ta and Zr-Hf fractionation ＂known＂ to take place during magmatism is fundamental for improved understanding of elemental fractionations through other earth processes in various tectonic environments, including the origin and evolution of continental crust, which has a characteristic subchondritic Nb/Ta value of -11-12.

关键词： Nb/Ta Zr/Hf correlated varia-tion magmatic process mass-dependent fractionation.

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magmatic Source Composition and Magmatism of the Volcanic Rocks in the Area of Kuruktag, Southern Xinjiang

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Continental Dynamics 2001年第1期6卷 51-60页

作者： JIANG Changyi, BAI Kaiyin, HI Aizhi, ZHAO Xiaoning and ZHANG Hongbo Xi抋n Engineering College, Xi抋n 710054, China Xi'an Engineering College Xi 'an 710054 China

In the Sinian-Cambrian strata in the area of Kuruktag, southern Xingjiang, four layers of volcanic rocks occurred in the Early Sinian Beiyixi Formation, Late Sinian Zhamoketi Formation and Shuiquan Formation, and Early Cambrian Xishanbulake Formation, respectively. Volcanics of the Shuiquan Formation and Xishanbulake Formation are of alkali basalt series, those of the Zhamoketi Formation are of alkali basalt series and tholeiite series, and those of the Beiyixi Formation are obviously characterized by bimodal assemblage and mostly belong to alkali volcanics. Multi-element distribution patterns of the rocks show continental tumescence characters of interplate basalt. Fractional crystallization of plagioclase led to negative Eu-anomalies of some volcanics and the cumulation of olivine resulted in high MgO and low SiO2 content of some volcanic rocks. The SiO2 saturability of volcanic rocks of the Xishanbulake Formation and Shuiquan Formation is lower than that of tholeiite of the Zhamoketi Formation. Correspondingly, the abundance of incompatible elements in the first two formations is higher than those in the last formation, and the differences can be attributed to the different degrees of partial melting. The intense fractionation of REE and the obvious depletion of HREE suggest that these volcanic rocks were derived from garnet lherzolite of the mantle in the continental lithosphere. The Ba/Nb, La/Nb, Ba/La, Ba/Th and Rb/Nb rations demonstrate that these volcanic rocks were exclusively derived from the enriched mantle, mainly the EMI type mantle.

关键词： volcanic rocks petrology, tectonic environment magmatic process mantle source Kuruktag southern Xinjiang

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