准噶尔盆地西北缘三叠系重矿物特征及其物源指示意义

Heavy mineral characteristics and its implication for provenance of the Triassic,NW Junggar

重矿物是物源变化极为敏感的指示剂。本次研究运用重矿物组合、重矿物百分含量统计分布、SPSS统计—因子分析及相关分析方法对准噶尔盆地西北缘三叠系地层中采集的20件砂岩、砂砾岩样品进行研究。通过重矿物特征及指示参数分析得到4类重矿物组合:锆石—电气石—锐钛矿—独居石—磷灰石组合,金红石—钛铁矿—白钛石—辉石组合,方铅矿—赤褐铁矿—黄铁矿组合以及磁铁矿—绿帘石—角闪石组合,推测母岩组合为中—酸性岩浆岩+沉积岩+变质岩;认为这套碎屑岩的源区为西北—北部的齐尔山—哈拉阿拉特山一带(属扎伊尔山前),物源为下石炭统太勒古拉组肉红色花岗岩,灰色—灰绿色安山岩、安山玢岩及灰色—灰黑色凝灰岩、粉—细砂岩,并且成岩显示为近源物质和远源物质混杂堆积。

Objectives: Heavy mineral is the extremely sensitive indicator of provenance. In NW Junggar,the provenance and the heavy minerals of the Triassic are studied. In order to identify the characteristics of heavy minerals and provenance position,this research applied several statistic methods to study 20 samples of sandstones and glutenite in northwestern margin of Junggar Basin. It is alsosignificant to the further illustration of the source-to-sink background.Methods: This research chose 5 profiles of well-developed outcrops to investigate,and collected glutenite and sandstone samples back. Several methods such as single mineral analysis,heavy mineral combinations,mineral percentage distributions,main factor analysis and correlation analysis are applied to research the types and characteristics of heavy minerals of Triassic. Then the source rocks and provenance are concluded.Results: According to the analysis of heavy mineral characteristics and parameters,18 heavy minerals are identified in the Middle to Upper Triassic samples in the NW Junggar,with most of the grains being angular to subangular and subangular to subround;the contents of zircon,tourmaline and iron ore was found to be commonly high. Among these minerals,there are two zircon types: a yellowish-pink type and a rose-colored type. Brown tourmaline is contained in samples,and a certain content of hornblende,mica debris,apatite,monazite and a few metamorphic minerals and alteration mineral clastics are also included, with the grains exhibiting partial limonitization. 4 heavy mineral combinations are classified in the NW Junggar: zircon-tourmaline-monazite-apatite-anatase, rutile-ilmenite-leucoxene-apatite-galenite, hematite-limonite-pyrite and magnetite-epidote-hornblende. The first two combinations are mainly composed of stable minerals,while the unstable mineral content of the latter two are relatively higher. According to the separation,grinding and heavy mineral stableness of the grains,it is concluded that most are probably from the near-provenance,although the sediments compose the near and distal deposits. There is large amount of columnar zircon,worm-shaped tourmaline of an automorphic to hemiautomorphic form exhibiting characteristics of magmatic origin. Due to the content of rutile,monazite,apatite,pyroxene,hornblende and abundant iron,especially magnetite and ilmenite,it is predicted that intermediate-acid magmatite is included in the source rocks. Subrounded to rounded sedimentary zircon and tourmaline indicate that sedimentary rocks are also included. Some symbol minerals of metamorphic rocks,such as garnet,allanite,epidote,and so on also suggest that a few metamorphic rocks are included.Conclusions: The provenance of the study area is the Qier-Halaalat Mountain area located northwest-north of the study area,belonging to the Front Zaire Mountain. The source rocks are composed of intermediate-acid magmatite,sedimentary rocks and metamorphic rocks from the Tailegula Formation,and are mainly intermediate-acid magmatite. The intermediate-acid magmatite probably consists of tuff,andesite,andesitic porphyrite and granite and granodiorite intrusive bodies. Siltstones and fine-grain sandstones are the main sedimentary source rocks.The metamorphic content may be caused by partial chloritization and epidotization from the regional dynamic metamorphism.