Petrology and Mineralogy of the I-Type Granites and Temperature-Pressure Conditions for Magma Formation in the Shahewan Rock Mass of the Qinling
详细信息
- 责任者：Liu Chunhua (State Key Laboratory of Continental Tectonics and Dynamics ; Institute of Geology ; Chinese Academy of Geological Sciences ; Beijing 100037 ; China) ; Wu Cailai
- 刊名：Geology and Prospecting
- 出版年：2013
- 卷期：49(4)
- 关键词：I型花岗岩 ; I-type granite ; 温压条件 ; P-T conditions ; 秦岭 ; Qinling Mountains
- 页码：p.595-608
- 图表：illus., 7 tables, 36 refs.
- 分类号：0801
- issnNo：ISSN0495-5331
- isbnNo：CN11-2043/P
摘要
The Shahewan rock mass, located in the southern Shangdan fault of the south Qinling, predominantly consists of quartz monzonite with the porphyroid-coarse grained texture. This paper studies the petrological and mineralogical characteristics, and discusses the magma-forming P-T conditions and the tectonic settings of the Shahewan rock mass, through petrologcial observations, chemistry total analysis, electronprobe microanalyses (EPMA) and scanned electron microscope (SEM) methods. The result demonstrates that the Shanhewan quartz monzonite belongs to high-K calc-alkaline I-type granite, with its SiO2 content varying from 66.40% to 70.08%,the Al2O3 content 14.36%～14.95%,the Fe2O3 content 0.93%～1.73%,the FeO content ranging from 1.22% to 1.65%, the CaO content 1.94%～2.78%,the(Na2O+K2O)content 7.88%～8.56%, the K2O/Na2O ratio 1.02～1.11 and the values of δ less than 3.3. The average LREE/HREE ratio is 13.63; the average (La/Yb)N ratio is 20.54; the average δEu and δCe are 0.89 and 0.95. The REE content averages 151×10-6,which is enriched in LREE and shows HREE depletion and slight negative Eu and Ce anomalies. In addition, the chondrite-normalized REE pattern of quartz monzonite belongs to the rightist type. Furthermore, the quartz monzonite is slightly rich in large ion lithophile elements such as Ba, K and Sr and depleted in high field strength elements such as Nb, Ce, Zr and Ti. The major rock-forming minerals are quartz, plagioclase(oligoclase to albite), K-feldspar, magnesiohornblende and magnesian biotite, whereas the accessory minerals include mainly zircon, sphene, magnetite, ilmenite and small amount of garnet. The average temperature of the initial magma is 755～780℃ and the crystallization temperature of the magma is 621～623℃. The depth of the magma emplacement is about 4 km and the pressure is about 1.26×108 Pa. The material source has crust-mantle mixed characteristics. The Shahewan quartz monzonite might have been formed in a setting with features of medium-temperature, water-rich and medium fugacity.
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