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Geoscience ›› 2020, Vol. 34 ›› Issue (05): 1067-1076.DOI: 10.19657/j.geoscience.1000-8527.2020.038

• Petrology • Previous Articles     Next Articles

Geochemistry Characteristics and Tectonic Environment Analysis of Middle Jurassic A-type Granites in Zhangjiawan Area, Heilongjiang Province

REN Yongjian1(), CHENG Shuo1(), ZHANG Mingming1, CAO Guangyuan2, YU Wang1, ZHAO Han1, LIANG Heng1, WANG Fuqiang1, QI Caiji1   

  1. 1. Geological Institute of China Chemical Geology and Mine Bureau,Beijing 100013,China
    2. China Chemical Geology and Mine Bureau,Beijing 100013,China
  • Received:2018-10-18 Revised:2020-05-06 Online:2020-10-28 Published:2020-10-29
  • Contact: CHENG Shuo

Abstract:

Through comprehensive petrographic and geochemical analyses of the Middle Jurassic syenite and alkali feldspar granite in Zhangjiawan area (Heilongjiang Province), we explore the regional granitic magmatism, genetic type and tectonic environment. The results show that the syenite granite and alkali feldspar granite have high SiO2(73.12%-77.82%), and are alkali-rich (K2O+Na2O = 6.36%-8.56%), Fe-rich and Mg-poor (FeO T/MgO = 6.67-25.43). The rocks have aluminum saturation index (A/CNK value) mostly above 1.1, distinct negative Eu anomalies (δEu = 0.01-0.74), enrichments in Th, U, K and Rb, and depletions in Ba, Sr, P and Ti. Zircon saturation temperatures are calculated to be 757-900 ℃, with an average of 816 ℃. The granites are A-type, and likely evolved from highly-fractionated granite to peraluminous A-type granite. REE features indicate that the magma was sourced from a mixture of mantle and lower crustal materials. The mid-Jurassic syenite granite-alkaline granite is A2-type, and was likely formed in a post-orogenic lithospheric extension setting. The above-mentioned lithogeochemical characteristics indicate that the Middle Jurassic regional tectonics was influenced by the west-dipping Paleo-Pacific subduction and post-collisional orogeny. The Zhangjiawan area was in a post-orogenic extension environment, and the lithosphere tectonic regime changed from compressive to extensional. The mantle upwelling accompanied by magma intrusion may have triggered the melting of crustal materials, which formed the syenite and alkali feldspar granite in the area.

Key words: Zhangjiawan area, Middle Jurassic, A-type granite, geochemistry, tectonic environment

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