The geology of the Aracruz pluton (Espírito Santo, Brazil), a Cambrian post-orogenic intrusion in the aftermath of Gondwana amalgamation

We present the first geological map of the Aracruz pluton (Espírito Santo, Brazil), which belongs to the late Cambrian post-orogenic suite that intruded the Nova Venécia metapelites and Ediacaran syn-orogenic S-type granites of the Araçuaí belt. Our mapping coupled with airbone magnetic and radiometric observations reveals that the pluton is not regularly zoned. It is composed of porphyric alkali feldspar granites, into which charnockite and quartz-diorite bands intruded as several pulses while the granites were a deforming crystal mush, and an undeformed central norite emplaced when most movement of the host magma had ceased. Steep-dipping concentric magmatic foliation, magmatic foliation parallel to tectonic foliation in the country rock, the noritic core, and the irregular charnockite and quartz-diorite band alignment suggest buoyancy-driven diapirism as emplacement mechanism. The Aracruz pluton is a superb example of mantle-derived post-orogenic intrusion attesting for the production of continental crust during the final stages of a Wylson cycle.

本研究首次完成了巴西圣埃斯皮里图州阿拉克鲁兹岩体（Aracruz Pluton）的首份地质填图。该岩体隶属于晚寒武纪后造山岩套（post-orogenic suite），侵入于阿拉瓜伊造山带（Araçuaí Belt）的新内韦西亚变泥质岩（Nova Venécia metapelites）与埃迪卡拉纪同造山S型花岗岩（S-type Granite）中。本次填图结合航空磁测与放射性观测数据，结果表明该岩体并不存在规则分带。岩体主体为斑状碱长花岗岩（porphyric alkali feldspar granites），在该花岗岩处于变形岩浆晶粥（crystal mush）阶段时，多期侵入脉冲形成的紫苏花岗岩（charnockite）与石英闪长岩（quartz-diorite）脉体穿插其中；而当寄主岩浆的大部分运移活动终止后，未受变形的中央苏长岩（norite）便完成就位。陡倾的同心状岩浆面理、与围岩构造面理平行的岩浆面理、苏长岩质核部，以及不规则排布的紫苏花岗岩与石英闪长岩脉体，均指示该岩体的就位机制为浮力驱动的底辟作用（diapirism）。阿拉克鲁兹岩体是幔源后造山侵入体的典型范例，证实了威尔逊旋回（Wilson Cycle）末期大陆地壳的生成过程。