Anatomy of the transpressional Dom Feliciano Belt and its pre-collisional isotopic (Sr–Nd) signatures: A contribution towards an integrated model for the Brasiliano/Pan-African orogenic cycle

The Dom Feliciano Belt evolution is reviewed based on cross-sections, space–time diagrams, P-T paths, and Sr–Nd isotopic data of pre-collisional metaigneous rocks. The belt is divided into northern, central and southern sectors, subdivided into tectonic domains, developed at Neoproterozoic pre-, syn- and post-collisional stages. The northern sector foreland pre-collisional setting represents a rift, with tholeiitic (meta)volcanic rocks (∼800 Ma) chronocorrelated to hinterland intermediate and acidic orthogneisses of high-K calc-alkaline arc signature. In contrast, the central sector records a complete section from the forearc towards the back-arc region during pre-collisional times. In the western domain, ophiolites (∼920 Ma) are associated with arc-related orthogneisses and metavolcanic rocks (880–830 Ma; 760–730 Ma). At back-arc position, continental arc-related magmatism (800–780 Ma) is registered by hinterland orthogneisses and central foreland metavolcanic rocks. Ophiolites on the hinterland opposite side comprise two compositional groups, with N-MORB and supra subduction signature, interpreted as a back-arc basin record (∼750 Ma). The pre-Neoproterozoic basement of the whole belt is correlated with the Nico Perez Terrane and Luis Alves Block (Archean to Mesoproterozoic, with Congo Craton affinity). This contrasts with the Piedra Alta Terrane (Rio de La Plata Craton, only Paleoproterozoic), westernmost Uruguay. The suture between the Piedra Alta and Nico Perez terranes is correlated with the suture zone in the westernmost central sector. Transpression affected both foreland and hinterland during collision (660–640 Ma), with high-T/low-P hinterland progressive exhumation, whilst foreland low- to medium-grade correlated sequences record underthrusting. Post-collisional processes included magmatism throughout the belt (640–580 Ma), strain partitioning along strike-slip shear zones, and foreland basin fill. Late tectono-metamorphic and magmatic processes (560–540 Ma) were attributed to the Kalahari Craton collision. Arc magmatism migration due to subduction angle variations suggests modern-style plate tectonics during Gondwana amalgamation. Diachronism and kinematic inversion are characteristic of an oblique convergent multi-plate orogenic system.     

义敦岛弧带弧后区板内岩浆作用的时代及意义

通过对义敦岛弧夏囊沟一带酸性火山岩的岩石地球化学和Rb-Sr同位素研究,首次在该岛弧弧后区确立了一条板内火山岩带.这些火山岩属钾玄岩系列,以高K2O,低CaO,富Nb、Ta、Zr、Hf,贫Sr、Eu为特点,具标准的"V"型稀土元素配分型式,构造环境判别图解显示其形成于板内裂陷环境.由4个全岩样品给出的Rb-Sr等时线年龄t=189.2±5.0 Ma,相关系数R=0.999?824,ISr=0.714?578.认为火山岩的成分分异受斜长石在岩浆源区熔融残留和岩浆形成后黑云母的结晶分离双重控制.位于该带东侧弧后扩张盆地中的晚三叠世勉戈组双峰火山岩的形成年龄为213.1 Ma,说明该岛弧仅仅在24 Ma左右时限内就完成了从俯冲造弧到板内裂陷的重大转换,由此可对该岛弧带的碰撞造山过程和演化时序做出精确约束.