皖南中生代两期花岗岩成因及深部过程

Two-stage Mesozoic granites are broadly cropped out in southern Anhui Province, the lower Yangtze region. As yet, petrogenesis and chronological framework about these granites are still ambiguous. There is not yet profoundly understanding about the geological evolutive processes reflected by that the early stage calc-alkaline I-type granite abruptly transformed into later stage alkaline A-type granite. Moreover, it is unclear about whether to take place and how to evolve for a series of deep geological processes, including upwelling of asthenosphere, underplating of basaltic magma and lithospheric delamination. The key accounting for these issues depends on detailed constraints on the origin of magma and petrogenesis as well as confirmation of the mode and evolutive sequence of deep geological processes which were corresponded by formation of these magmatic rocks under a tectonic setting of transformation from contraction to extension. Aimed to deciphering the petrogenesis and magmatic process, ascertaining the characteristics of magmatic source and exploring the deep geological processes, this work will focus on the integrated petrologic, geochemical and zircon U-Pb chronological and Hf-O isotopic studies on the Mesozoic magmatic rocks with different types and ages from southern Anhui. Accessorily, the zircon chronological and isotopic studies on the Precambrian metamorphic basement will also be concerned. Finally, by systematically contrasting with research about the magmatic rocks from the area along the Yangtze River, the continental dynamic evolution of different tectonic units in the lower Yangtze region in later Mesozoic will be researched and credibly constrained.

下扬子皖南地区广泛发育两期中生代花岗岩，其年代学格架和成因还不清晰，从早期钙碱性I型花岗岩为主突变至晚期碱性A型花岗岩为主，所反映的地质演化过程尚无深入认识，对由挤压转为拉张的构造背景下，软流圈上涌、玄武岩浆底侵、岩石圈下部拆沉、壳幔相互作用等一系列深部过程发生与否和演化规律还不清晰。解决这些问题的关键是对岩浆物质来源和岩石成因的详细制约，以及岩石形成所对应的深部地质作用发生的方式和演化顺序的厘定。本项目拟对皖南地区中生代各种类型、不同时代的岩浆岩开展系统的岩石学、地球化学、锆石U-Pb 年代学和Hf-O同位素联合示踪研究，辅以前寒武纪变质基底锆石年代学和同位素分析，深入辨析岩石成因和岩浆过程，厘定岩浆源区性质，剖析各种深部地质作用发生的过程和方式。进而与下扬子沿江地区岩浆岩的研究成果进行系统对比，为下扬子地区不同构造单元晚中生代时期大陆动力学演化提供可靠的制约。

下扬子皖南地区和长江中下游地区以及大别造山带均广泛发育中生代花岗岩，其年代学格架和成因还不清晰，从早期至晚期岩性的变化所反映的深部过程尚无深入认识。本次工作首先针对皖南地区13个复式岩基和10多个小岩体开展了岩石学、全岩地球化学、锆石原位Hf-O同位素以及锆石年代学的系统研究，建立了区内三阶段精细年代学格架，揭示了从早到晚，岩浆作用从高钾钙碱性中酸性I型花岗岩向高钾钙碱性A性花岗岩转变的时空演化规律。早阶段岩浆起源以中-新元古代增生地壳物质为主，有少量交代岩石圈地幔部分熔融岩浆的加入；第二阶段岩浆岩主要为类似早阶段岩浆经历AFC过程形成；而晚阶段A型花岗岩为经历早期部分熔融岩浆抽取后，残留地壳再次深熔形成。从早到晚，岩浆源区从加厚下地壳演化至减薄后的地壳，对应了古太平洋板块向中国东部大陆的俯冲以及俯冲角度的改变。对比研究表明，长江中下游地区中生代经历了类似于皖南地区的构造-岩浆演化过程，但前者第一和第二阶段岩浆岩中岩石圈地幔物质的贡献更为显著，且存在更晚期的岩浆作用。大别造山带燕山期岩浆岩作用总体时空格架和下扬子类似，但持续时间更晚，岩浆作用以地壳深熔作用为主，源区地壳更加古老。从成矿的角度，壳幔混合岩浆的成矿以Cu-Pb-Zn-Au多金属成矿为主，幔源岩浆的成矿以铁矿为主，主要发育在长江中下游地区；而皖南地区和大别造山带的中生代岩浆岩以壳源为主，成矿主要以Mo-W-Au多金属为主，分别表现出不同的成矿专属性。这三个构造单元燕山期岩浆活动对应统一的构造动力学背景，但由于早先构造区域的差异（不同时期的造山带，前陆），造成在成岩成矿方面也存在一定的差异性。