Dados da Submissão

Título

Amphibole rich clots and oxygen isotope as evidence of metabasaltic oceanic crost in two Ediacaran epidote + diopside-bearing granitoids in Northeastern Brazil

Texto do resumo

Low-grade metamorphic rocks of the Cachoeirinha-Salgueiro Terrane, northeastern Brazil, were intruded by late Cryogenian-Ediacaran magmatic-epidote bearing tonalites to granodiorites, occasionally carrying clinopyroxene. Among them, the adjacent Olho d'Água and Tamboril stocks exhibit amphibole-rich clots (ARC) composed of tremolite or fibrous actinolite and/or Mg-hornblende. These clots seldomly exhibit nematoblastic texture and preserved igneous texture instead are usually observed. They are commonly surrounded by a biotite, hornblende + biotite or, less often, by clinopyroxene layer that armor them, limiting the interaction between ARC and host magma. Clinopyroxene partially transformed into amphibole ± biotite within ARCs or as patches in the hosts, surrounded by tremolite overgrown by hornblende is, perhaps, a relict from basalt–amphibolite transformation. Zircon U-Pb dating yielded an age of 624 ± 6.0 Ma for the Tamboril stock and 611 ± 3.0 Ma for the Olho d’Água one. Nd model age (tDM) ranges from 1.0 to 1.2 Ga in the Tamboril stock and is of 1.2 Ga in the Olho d'Água, while εNd(0.6 Ga) values vary from 0.24 to -1.88 in the fomer stock and from -1.50 to -1.56 in the latter. Back-calculated initial 87Sr/86Sr ratios vary from 0.707 to 0.711 in both plutons. δ18O(qz) values range from 9.9 to 14.2‰, δ18O(cpx) from 8.9‰ to 13.2‰ and δ18O(bi) from 6.7‰ to ~10‰. δ18O distribution patterns within the plutons for qz, cpx, bi and qz₋bi show that values increase towards pluton margins and may reflect post-crystallization interaction between the plutons and water from sedimentary rocks that covered the plutons borders. ARC(wr) δ18O as well as pluton δ18O(wr) values estimated from δ18O(zr), δ18O(qz) or δ18O(cpx are greater than10‰. ARCs are probably fragments from an amphibole-rich metabasaltic source that underwent low-T interaction with oceanic water. The Sr-Nd-O isotopes for ARCs and host support a metabasaltic source for the magmas with further crustal contamination.

Palavras Chave

magmatic epidote; amphibole-rich clots; oxygen isotopes; Sr and Nd isotopes; northeastern Brazil