Petrogenesis, U-Pb and Sm-Nd geochronology of the Furna Azul Migmatite: partial melting evidence during the San Ignácio Orogeny, Paraguá Terrane, SW Amazon Craton

ABSTRACT: The Furna Azul Migmatite is a ~10 km2 complex located in Pontes e Lacerda city, Mato Grosso, Brazil. It belongs to Paraguá Terrane, limit with Rio Alegre Terrane, southeast of San Ignacio Province, in Amazon Craton. It consists of transitional metatexites with amphibolite enclaves and dioritic injections. The rocks were divided in residuum rich and leucosome rich; both have three deformation phases marked by folded stromatic layers affected by spaced foliation and metamorphosed in amphibolite facies, represented by garnet, biotite, sillimanite, and by the clinopyroxene in the enclaves. The metamorphic retrograde to greenschist is marked by formation of chlorite, muscovite and prehnite. Residuum-rich metatexites show higher CaO and Na2O contents, separating them from K2O, Ba and Rb enriched transitional metatexites. U-Pb on zircon and Sm-Nd whole-rocks dating indicates that the residuum-rich metatexite crystallized at 1436 ± 11 Ma, with a TDM age of 1.90 Ga and εNd(1.43) of -0.54, whereas the dioritic injection crystallized at 1341,7 ± 17 Ma with a TDM age of 1.47 Ga and εNd(1.34) of 3.39. These results indicate that the Furna Azul Migmatite protolith was formed during the San Ignácio Orogeny and was reworked during the same orogeny, as basement for collisional to post-magmatic granites from Pensamiento Intrusive Suite.

摘要：弗纳尔阿祖尔混合岩（Furna Azul Migmatite）是一处面积约10 km²的地质复合体，产于巴西马托格罗索州蓬蒂斯埃拉塞达市，隶属于亚马逊克拉通（Amazon Craton）圣伊格纳西奥省（San Ignacio Province）东南部的帕拉瓜地体（Paraguá Terrane），与里约阿莱格里地体（Rio Alegre Terrane）接壤。该复合体由发育角闪岩包体与闪长质注入体的过渡型深熔混合岩（metatexite）构成。研究将其划分为富残余体与富淡色体两类，二者均记录有三期变形作用，表现为受间隔面理改造的褶皱条层状构造，且均经历了角闪岩相变质作用，以包体中的石榴子石、黑云母、矽线石及单斜辉石作为标志性矿物。该混合岩的退变质作用达绿片岩相，以绿泥石、白云母与葡萄石的形成为典型标志。富残余体深熔混合岩具有较高的CaO与Na₂O含量，与富K₂O、Ba及Rb的过渡型深熔混合岩形成显著区分。锆石U-Pb测年与全岩Sm-Nd测年结果显示，富残余体深熔混合岩的结晶年龄为1436 ± 11 Ma，亏损地幔模式年龄（TDM age）为1.90 Ga，εNd(1.43)值为-0.54；而闪长质注入体的结晶年龄为1341.7 ± 17 Ma，亏损地幔模式年龄为1.47 Ga，εNd(1.34)值为3.39。上述结果表明，弗纳尔阿祖尔混合岩的原岩形成于圣伊格纳西奥造山运动（San Ignácio Orogeny），并在同一造山事件中发生了重熔改造，可作为彭萨门托侵入岩套（Pensamiento Intrusive Suite）中碰撞型至后岩浆型花岗岩的基底。