U-Pb Zircon (SHRIMP) ages of granite sheets and timing of deformational events in the Natal Metamorphic Belt, southeastern Africa: Evidence for deformation partitioning and implications for Rodinia reconstructions

摘要

This study provides constraints on the ages of deformation events and fabric development in deformed rocks of the Margate Terrane of the Natal Metamorphic Province. The Margate Terrane forms the south-ernmost of three terranes considered to represent multiple arc accretion onto the southern margin of the Kalahari Craton, and geochronological data indicate that the Margate Terrane has a long history of sporadic magmatism from similar to 1180 to similar to 1025 Ma. Two granite sheets of differing structural age, as revealed by deformational fabric and cross-cutting relationships, were sampled for U-Pb (SHRIMP) dating from coastal outcrop at Southbroom (30 degrees 54'43.61 '' S, 30 degrees 20'1.61 '' E). The older sheet contains fabrics related to both D1 and D2 events, whereas the younger shows evidence for syn-D2 emplacement and contains an S2 fabric. For both intrusive sheets, zircon core domains showing magmatic zoning yielded ages that are statistically identical at 1075 +/- 6 Ma. This is interpreted to represent the intrusion age of the older sheet, and to sampling of a xenocrystic population in the cross-cutting sheet due to assimilation. Zircons from both sheets show metamorphic rim zones with a mean age of 1042 +/- 10 Ma, which is attributed to rim growth during development of the S2 foliation, close to the intrusion age of the younger sheet. The 1075 +/- 6 Ma age is comparable to the ages of other granitic units in the Margate Terrane that intruded between 1091 and 1070 Ma and implies that all of these predated the D1 deformation, which is considered to record the accretion of the terrane onto the Kalahari Craton. The D2 event is characterized by northward-verging folds with a pervasive southward-dipping axial planar fabric S2, which largely overprinted the S1 fabric. Previously, the pure-shear D2 deformation was considered to be older than the narrow sinistral shear zones occurring within the Mzumbe and Margate Terranes. However, dating of the D2 event at 1042 +/- 10 Ma indicates that these deformations are coeval, and represent deformation partitioning during a transpressional event at similar to 1050-1025 Ma that may have been related to collision with Laurentia during the amalgamation of Rodinia. (C) 2016 Elsevier B.V. All rights reserved.