华北克拉通北缘显生宙含石榴石淡色花岗岩:特征、时代及成因初探

淡色花岗岩是一种特殊的花岗岩类型,其暗色矿物含量低,且多含有白云母、电气石或石榴石等富铝矿物。通常认为,淡色花岗岩是大陆碰撞造山带最具标志性的岩石类型,主要来自于地壳内沉积物的部分熔融,虽然一些最新研究强调它只是岩浆高度分异与演化的产物。显生宙时期,华北克拉通北缘花岗质岩浆作用强烈,然而对于区域存在的少量具有淡色花岗岩特征的岩体却关注较少。本文在对冀东麻地含石榴石白云母二长花岗岩详细研究的基础上,对华北北缘显生宙含石榴石淡色花岗岩的特征、时代进行了总结与对比,并对其成因进行了初步探讨。结果表明,这些岩石具有淡色花岗岩的典型矿物组成与地球化学特征,但已有数据还不足以充分论证其是否由变沉积岩部分熔融形成。与此同时,稀土元素四分组效应与Nb/Ta、Zr/Hf、Y/Ho、K/Rb、Rb/Sr等比值以及CaO、Ba、Sr等元素含量的差异性指示,这些花岗岩经历了不同程度的岩浆分异作用,部分岩石受到了岩浆演化晚期熔-流体相互作用的影响。岩浆分异程度的不同,是导致这些淡色花岗岩具有不同稀有金属成矿潜力的重要因素,高度分异演化有利于成矿元素的富集。由于仅部分淡色花岗岩经历了较高程度的分异,因此岩浆分异可能并非淡色花岗岩形成的必要机制。华北北缘含石榴石淡色花岗岩集中出现在中-晚二叠世、中-晚侏罗世。前者的形成,与古亚洲洋闭合时的碰撞造山有关;后者与东北、华南等中国东部相似岩石同时代产出,形成于古太平洋俯冲的大地构造背景下,其地球动力学内涵值得进一步探讨。

Phanerozoic garnet-bearing leucogranite in the northern margin of North China Craton: Characters, timing and preliminary petrogenesis study

Leucogranites, which usually contain very few mafic minerals but with muscovite, tourmaline or garnet, are characteristic igneous rocks in collisional orogens. They were frequently proved to be the products of crustal paragneiss melting, although extensive magmatic differentiation and later evolution was proposed dominantly in their genesis recently. In this paper, we investigate the Phanerozoic garnet-bearing leucogranites, which has been ignored mostly, in the northern margin of North China Craton (NCC). Taken the Madi stock as an example, the recognized leucogranites in the northern NCC are proved to be typical leucogranite with high silica and A/CNK ratios. They have similar mineral assemblage and geochemical composition with the Manaslu Himalayan leucogranite which was alleged to be S-type granite. The garnets also have prevailing almandine-spessartine composition, which also resembles those in other previously reported leucogranite in the world. However, the available whole rock Sr-O-Nd isotopes and P₂O₅ contents make their derivation controversial. Their sccattered CaO/Na₂O, Rb/Sr and Rb/Ba ratios can''t be confidently interpreted by systematic compositional difference of the sedimentary provenance. Instead, the variable tetrad REE patterns and deviation of Nb/Ta, Zr/Hf, Y/Ho, K/Rb and Rb/Sr ratios from the common chondrite and granite imply undoubtedly different degree of magma fractional crystallization with later melt-fluids interaction. The most highly evolved leucocratic magma in the region, which is represented by the Madi stock in which columbite and topaz are found here, is economically mineralized. Absence of significant magmatic differentiation in some garnet-bearing leucogranites (such as the Fangniugou, Houzhuo and some Jiangjia samples) suggests that not all leucogranites are high-evolved granite. Thus, the magma evolution process may contribute to just some imprints of the leucogranites but are not necessary in their genesis. Despite, the highly differentiaon processes, together with the enrichment of fluorine in the late evolutional stage, are crucial to the rare metal mineralization. Temporally, the Phanerozoic garnet-bearing leucogranites appeared in both mid-late Permian and mid-late Jurassic. The former may be related to the collisional process between NCC and the adjacent blocks during the closure of Paleo-Asian Ocean, while the latter seem to be formed contemporaneously with the leucogranite found in South China and Northeastern China area when the Paleo-Pacific ocean plate was subducting beneath the NCC. Further exploration on geodynamics is still needed for the Jurassic ones. So the study promotes our understanding on the petrogenesis of leucogranite and their relationship with rare-metal mineralization and regional tectonic evolution.