The Late Cretaceous Mamba granodiorite belongs to a part of the Mesozoic Gangdese continental magmatic belt. No quantitative mineralogical study has been made hitherto, and hence the depth at which it formed is poorly...The Late Cretaceous Mamba granodiorite belongs to a part of the Mesozoic Gangdese continental magmatic belt. No quantitative mineralogical study has been made hitherto, and hence the depth at which it formed is poorly constrained. Here we present mineralogical data for the Mamba pluton, including host rocks and their mafic microgranular enclaves（MMEs）, to provide insights into their overall crystallization conditions and information about magma mixing. All amphiboles in the Mamba pluton are calcic, with ~B（Ca＋Na）〉1.5, and Si=6.81-7.42 apfu for the host rocks and Si=6.77-7.35 apfu for the MMEs. The paramount cation substitutions in amphibole include edenite type and tschermakite type. Biotites both in the host rocks and the MMEs collectively have high Mg O（13.19 wt.%-13.03 wt.%） contents, but define a narrow range of Al apfu（atoms per formula unit） variations（2.44-2.57）. The oxygen fugacity estimates are based on the biotite compositions cluster around the NNO buffer. The calculated pressure ranges from 1.2 to 2.1 kbar according to the aluminum-in-hornblende barometer. The computed pressure varies from 0.9 to 1.3 kbar based on the aluminum-in-biotite barometer which corresponds to an average depth of ca. 3.9 km. Besides, the estimates of crystallization pressures vary from 0.8 to 1.4 kbar based on the amphibole barometer proposed by Ridolfi et al.（2010）, which can be equivalent to the depths ranging from 3.1 to 5.2 km. The MMEs have plagioclase oscillatory zonings and quartz aggregates, probably indicating the presence of magma mixing. Besides, core-to-rim element variations（Rb, Sr, Ba, and P） for the K-feldspar megacrysts serve as robust evidence to support magma mixing and crystal fractionation. This indicates the significance of the magma mixing that contributes to the formation of K-feldspar megacryst zonings in the Mamba pluton.展开更多
The Quxu （曲水） complex is a typical intrusive among the Gangdese batholiths. Two sets of samples collected from the Mianjiang （棉将） and Niedang （聂当） villages in Quxu County, including gabbro, mafic micro-enc...The Quxu （曲水） complex is a typical intrusive among the Gangdese batholiths. Two sets of samples collected from the Mianjiang （棉将） and Niedang （聂当） villages in Quxu County, including gabbro, mafic micro-enclaves （MME）, and granodiorites in each set, were well dated in a previous SHRIMP zircon U-Pb analysis （47-51 Ma）. In this article, the same zircons of the 6 samples were applied for LA ICP-MS Hf isotopic analysis. The total of 6 samples yields 176Hf/177Hf ratio ranging from 0.282 921 to 0.283 159, corresponding to εHf（t） values of 6.3-14.7. Their Hf depleted-mantle modal ages （TDM） are in the range of 137-555 Ma, and the zircon Hf isotope crustal model ages （TDMC） range from 178 to 718 Ma. The mantle-like high and positive Era（t） values in these samples suggest a mantledominated input of the juvenile source regions from which the batholith originated. The large variations in εHf（t） values, up to 5-ε unit among zircons within a single rock and up to 15-ε unit among zircons from the 6 samples, further suggest the presence of a magma mixing event during the time of magma generation. We suggest that the crustal end-member involved in the magma mixing is likely from the ancient basement within the Lhasa terrane itself. The zircon Hf isotopic compositions further suggest that magma mixing and magma underplating at about 50 Ma may have played an important role in creating the crust of the southern Tibetan plateau.展开更多
基金funded by the National Natural Science Foundation of China (Nos. 41403028, 40830317)the China Postdoctoral Science Foundation (No. 2015T80113)+1 种基金China University of Geosciences (No. GMPR201509)the Fundamental Research Funds for the Central Universities of China (No. 2652015018)
文摘The Late Cretaceous Mamba granodiorite belongs to a part of the Mesozoic Gangdese continental magmatic belt. No quantitative mineralogical study has been made hitherto, and hence the depth at which it formed is poorly constrained. Here we present mineralogical data for the Mamba pluton, including host rocks and their mafic microgranular enclaves（MMEs）, to provide insights into their overall crystallization conditions and information about magma mixing. All amphiboles in the Mamba pluton are calcic, with ~B（Ca＋Na）〉1.5, and Si=6.81-7.42 apfu for the host rocks and Si=6.77-7.35 apfu for the MMEs. The paramount cation substitutions in amphibole include edenite type and tschermakite type. Biotites both in the host rocks and the MMEs collectively have high Mg O（13.19 wt.%-13.03 wt.%） contents, but define a narrow range of Al apfu（atoms per formula unit） variations（2.44-2.57）. The oxygen fugacity estimates are based on the biotite compositions cluster around the NNO buffer. The calculated pressure ranges from 1.2 to 2.1 kbar according to the aluminum-in-hornblende barometer. The computed pressure varies from 0.9 to 1.3 kbar based on the aluminum-in-biotite barometer which corresponds to an average depth of ca. 3.9 km. Besides, the estimates of crystallization pressures vary from 0.8 to 1.4 kbar based on the amphibole barometer proposed by Ridolfi et al.（2010）, which can be equivalent to the depths ranging from 3.1 to 5.2 km. The MMEs have plagioclase oscillatory zonings and quartz aggregates, probably indicating the presence of magma mixing. Besides, core-to-rim element variations（Rb, Sr, Ba, and P） for the K-feldspar megacrysts serve as robust evidence to support magma mixing and crystal fractionation. This indicates the significance of the magma mixing that contributes to the formation of K-feldspar megacryst zonings in the Mamba pluton.
基金supported by the National Basic Research Program of China (Nos. 2009CB421002, 2002CB412600)the Na-tional Natural Science Foundation of China (Nos. 40873023, 40830317, 40672044, 40503005, 40572048, 40473020)+1 种基金111 Project (No. B07011)China Geological Survey (No. 1212010610104)
文摘The Quxu （曲水） complex is a typical intrusive among the Gangdese batholiths. Two sets of samples collected from the Mianjiang （棉将） and Niedang （聂当） villages in Quxu County, including gabbro, mafic micro-enclaves （MME）, and granodiorites in each set, were well dated in a previous SHRIMP zircon U-Pb analysis （47-51 Ma）. In this article, the same zircons of the 6 samples were applied for LA ICP-MS Hf isotopic analysis. The total of 6 samples yields 176Hf/177Hf ratio ranging from 0.282 921 to 0.283 159, corresponding to εHf（t） values of 6.3-14.7. Their Hf depleted-mantle modal ages （TDM） are in the range of 137-555 Ma, and the zircon Hf isotope crustal model ages （TDMC） range from 178 to 718 Ma. The mantle-like high and positive Era（t） values in these samples suggest a mantledominated input of the juvenile source regions from which the batholith originated. The large variations in εHf（t） values, up to 5-ε unit among zircons within a single rock and up to 15-ε unit among zircons from the 6 samples, further suggest the presence of a magma mixing event during the time of magma generation. We suggest that the crustal end-member involved in the magma mixing is likely from the ancient basement within the Lhasa terrane itself. The zircon Hf isotopic compositions further suggest that magma mixing and magma underplating at about 50 Ma may have played an important role in creating the crust of the southern Tibetan plateau.
