扬子陆块西北缘米仓山早新元古代碱性侵入岩杂岩带地球化学研究

Ultrabasic to intermediate alkaline and carbonatitic intrusions comprise the alkaline complex province or belt in the Micangshan terrain along the northwestern margin of the Yangtze block. Our preliminary study reveals that they formed at the early Neoproterozoic of ~875 Ma, rather than the early Paleozoic or Sinian as has long been regarded. Due to actuality that the documented works on the origin of these alkaline complexes, the nature of the deep mantle source and tectonic setting and that the early Neoproterozoic alkaline magmatism is of great scientific significance for our understanding of the unification process of the South China block, the alkali complexes are selected to be the objects of this research project to be applied. A geochronological study based on in situ U-Pb dating of minerals of zircon, apatite, monazite and titanite is to be conducted, to understand the timing and possible episodes of the alkaline magmatism of the region. To understand the geochemical characteristics of the intrusions, the nature of their mantle sources, composition of the primary magma and the relationship of its evolution with the origin of the alkali rock series, comprehensive analyses of elemental and Sr-Nd radiogenetic and C-O-Mg stable isotopes of whole rocks and minerals are to be carried out. Based on the distribution of highly incompatible elements between the silicatic and carbonatitic intrusions and isotopic signatures, the geodynamic mechanism of the metasomatism of recycled crustal materials to the mantle rocks which sourced the alkali magma is to be explored. Coupled with our and others’ previous works of the region, the tectonomagmatic evolution and nature of the region along the northwestern margin of the Yangtze block are to be discussed, and accordingly, to provide new insights into the understanding of unification process of South China and its relationship with the assembly and breakup of Rodinia supercontinent.

超基性-中性碱性岩与碳酸岩组成的杂岩带分布于扬子陆块西北缘米仓山系，曾被视为早古生代或震旦纪形成。我们的预研究表明，其形成年龄应为新元古代~875Ma。基于目前对这些碱性杂岩的成因、深部地幔源区性质和构造意义等仍缺乏了解的现状及其对认识华南陆块的统一过程具有重要的科学意义，本申请项目将以这些碱性杂岩为研究对象，采用多矿物U-Pb同位素定年方法，对其形成时代和可能的期次进行高精度厘定；采用全岩和矿物微区元素、Sr-Nd同位素和C-O-Mg稳定同位素示踪研究，了解各侵入体的地球化学特征、地幔源区性质、初始岩浆的化学组成及其演化与不同岩性碱性岩的成因关系；据强不相容元素在碱性硅酸盐和碳酸盐岩浆间的分配行为和同位素特征，分析再循环地壳物质对碱性岩地幔源岩发生交代作用的动力学机制；结合原有研究积累，探讨扬子陆块西北缘新元古代构造-岩浆事件的性质和演化特点，为更全面地认识统一华南陆块的形成过程及其与Rodinia超大陆演化的相互关系提供新线索。

扬子陆块西北缘米仓山碱性杂岩带分布于四川旺苍-南江，由火成碳酸岩和超基性-基性-正长岩碱性侵入体组成，可分为坪河、水磨、中子园和李家河四个单元。通过系统的同位素年代学研究，厘定了区内碱性杂岩带侵入体形成于两个时期，分别为~870Ma的坪河杂岩体、水磨正长岩和李家河碳酸岩以及~760Ma的水磨超基性-基性碱性岩体、方解石辉石岩-碳酸岩和中子园杂岩体。这些年龄值明显区别于早期发表的结果。各单元侵入体不同程度发育堆晶结构，其中坪河杂岩体主要由钛铁霞辉岩、霓霞岩和磷霞岩组成，水磨杂岩体由碱性辉石岩、辉长岩、斜长角闪石岩、碳酸岩和正长岩组成，中子园杂岩体主要为碱性辉长岩和碱性角闪辉长岩，局部为次火山岩(玄武岩)，而李家河单元由单一的碳酸岩组成。在矿物成分上，辉石主要为钛次透辉石，少数为霓石或霓辉石；碳酸盐均为方解石，长石主要为钠长石，少数为倍长石；Ti-Fe氧化物主要由钛铁矿和磁铁矿组成，磷灰石均为氟磷灰石，橄榄石为镁橄榄石。.元素地球化学特征揭示，坪河杂岩体由同源岩浆经结晶分异形成，水磨杂岩体基性-超基性硅酸岩与碳酸岩脉为不混熔岩浆经结晶分异形成，但正长岩由更早期岩浆作用形成；中子园杂岩体由多端元或不均一幔源岩浆形成，而李家河岩体由单一碳酸盐岩浆形成。形成旺苍-南江杂岩带不同单元碱性岩的岩浆均经历了显著的化学演化，故难以用于反演初始岩浆组成和源区性质。.李家河-水磨碳酸岩的Sr-Nd同位素组成接近于HIMU端元，~870Ma的坪河杂岩体和水磨正长岩则相反，除HIMU端元外，还存在EMI和EMII端元的参与；~760Ma的水磨和中子园基性-超基性岩样品同位素组成与碳酸岩相近，但εNd(t)略低，水磨样品具更低的Sr同位素比值，而中子园样品Sr同位素比值略高。.本次工作认为，区内新元古代碱性岩浆作用发生于大陆边缘裂谷环境，经历了多期岩浆事件叠加，与米仓山-汉南新元古代岩浆岩带共同组成了扬子西北缘大火成岩省，其时限和背景明显区别于形成于陆内环境、与地幔柱关系密切的大多数显生宙LIP。