岩浆混合作用的类型、标志、机制、模式及其与成矿的关系——以新疆北部为例

新疆北部广泛发育具有岩浆混合特征的岩体,岩体类型主要有钙碱性花岗岩、碱长花岗岩和碱性花岗岩,其中以第一类最为发育;岩体中的暗色包体类型以闪长质为主,成分可从闪长岩、石英闪长岩、石英二长闪长岩,过渡到花岗闪长岩,部分地区可发育辉长质、安山玄武质、安山玢岩质包体;有些地区还发育有岩浆混合岩.该区岩浆混合作用主要形成于四个大地构造演化阶段,即D3-C1交界的洋-陆俯冲期、C1末-C2初的碰撞期、C2-P的后碰撞期和T1的板内期.岩浆混合作用的判别标志有岩体宏观标志(近等轴状)、野外宏观标志(暗色微粒包体发育,有时可见岩浆混合岩)、显微标志(淬冷和矿物不平衡结构)、岩石地球化学标志(线性过渡特征)、年龄(一致)标志等.该区的岩浆混合作用是多级次的,其深部岩浆源区的混合表现为壳幔相互混合作用;混合作用表现出多阶段性,以基性岩浆注入到酸性岩浆为主要方式,由物理混合到化学混合渐变过渡.该区岩浆混合作用与斑岩铜钼金矿床密切相关,其成矿关系模式可概括为:基性岩浆与花岗质岩浆混合产生混合岩浆,经过进一步分异演化形成了含矿岩浆;与岩浆混合作用有关的斑岩铜钼矿的找矿标志为:岩浆混合作用强烈的大岩体内或附近晚期补充侵入的小型斑岩体具有找矿潜力.

Type, indicator, mechanism, model and relationship with mineralization of magma mixing: A case study in North Xinjiang

Magma mixing rockbodies are widely developed in the North Xinjiang. The sorts of rockbody can be classified into three types: calc-alkaline granitoid, alkali-feldspar granite and alkali granite, among which the first type is dominant. The microgranular enclaves type is dominantly dioritic having viarable composition from diorite, quartz diorite, quartz monzodiorite to granodiorite, with gabbro, andesitic basalt and andesitic porphyrite in some cases. Even magma-mixed rocks occur in some area. Magma mixing in the study area develops in four geological/tectonic stages: ocean-continent subduction stage in D₃-C₁, continent-continent collision stage in C₁-C₂, post-collision stage in C₂-P₂ and intra-plate stage in T₁. Discriminated indicators for magma mixing can be summed up as rockbody occurrence (e.g. in roughly equiaxial shape), field indicator (e.g. presence of various MME and occasional magma-mixed rocks), microscopic indicator (e.g. quenching and petrographic disequilibrium texture), geochemical indicator (e.g. linear and transition composition and geochemical parameter), and geochronology indicator (accordant age of related rocks). It is speculated that the magma mixing in North Xinjiang should be multi grades/orders, and the deep mixing of magma source behaves as crust-mantle mixing. The magma mixing mechanism and model are characterized by multi-stages, injected manner of acid magma by basic magma, and sequential physic-chemic mixing. It is closely relevant to porphyry Cu-Mo-Au mineralization, as the model that basic-acid mixed magmas evolve to ore-bearing magma through differentiation. Therefore, the prospecting indicator for the porphyry deposit in North Xinjiang may be ascribed to that the late supplying intruded small porphyry stock neighboring, surrounding and within magma mixing granitoid batholith.