History of volcanism and sedimentation synchronous with plutonism during Rhyacian in Serra das Pipocas Greenstone Belt, Borborema Province, NE Brazil

Abstract

The Serra das Pipocas Greenstone Belt is one of the rare Paleoproterozoic greenstone belts within the Borborema Province. It is on the west border of the Archean/Paleoproterozoic Nucleus (Cruzeta and Mombaça Complex) of the Ceará Central Domain, northern portion of the Borborema Province. The Serra das Pipocas Greenstone Belt has an elongated sigmoidal shape encompassing about 1000 km2 of the Ceará Central Domain. It consists of a metavolcanosedimentary sequence formed by metasedimentary psammitic-pelitic-marly rocks and interleaved metaultramafic and metamafic rocks, represented by chlorite-anthophyllite-actinolite/tremolite schists and amphibolites. Layers of metatuffs, metabasic, meta-acid sills (metadacites), metacherts, gondites and banded iron formations occur within the metamafic rocks. Actinolite, metagabbro, metadiorite bodies, metabasic dike, Neoproterozoic metagranodiorites and leucogranites crosscut the metavolcanosedimentary sequence. Close to the borders of metagranodioritic bodies (zircon U–Pb 2181 ± 4.4 Ma), the metavolcanosedimentary sequence is hydrothermally altered. The metaultramafic rock (chlorite-anthophyllite-actinolite/tremolite schist) contains high MgO (>18 wt%), low alkali (≤1 wt%) and TiO2 (<0.8 wt%) and Al2O3/TiO2 ratios between 9.94 and 27.26, with a mean value of 17.13. Values of (Gd/Yb)N vary between 1.00 and 1.88, which attest affiliation with komatiite protoliths, between the Munro and Barberton types. The amphibolites are usually fine-grained and depleted in ΣREE (33.14–45.02 ppm), with low (La/Yb)CH values (0.74–3.82). This reflects weak LREE enrichment in relation to HREE, and no Ce and Eu anomalies [(Eu/Eu*)n between 0.98 and 1.04]. Such patterns are similar to T-MORB (1.7–4.3). Additionally, some mafic-ultramafic metaplutonic rocks present chemical composition similar to their volcanic counterparts, and this may indicate cumulates or sills contemporary to this sequence. Zircon U–Pb dating using LA-ICP-MS of three distinct metadacite occurrences (western border: 2234 ± 13 Ma, eastern border: 2212 ± 13 Ma, and central portion: 2156 ± 45 Ma) and metagranodiorites (2181 ± 4.4 Ma) indicates that the sequence was formed during the Rhyacian/Orosirian and later affected by late Neoproterozoic.