Evolution of the Solonker suture zone: Constraints from zircon U-Pb ages, Hf isotopic ratios and whole-rock Nd-Sr isotope compositions of subduction- and collision-related magmas and forearc sediments
Journal
Journal of Asian Earth Sciences
Journal Volume
34
Journal Issue
3
Pages
245-257
Date Issued
2009
Author(s)
Abstract
The Solonker zone in northern Inner Mongolia (China) is considered as the suture between the North China Craton and the South Mongolian microcontinent. Two magmatic belts are recognized along the suture zone: a subduction-related magmatic belt (represented by the Baolidao arc rocks), and a younger, collision-related granite belt (represented by the Halatu granites). We use zircon U-Pb ages, zircon in-situ Hf isotopic analyses and whole-rock Nd-Sr isotopic data of the two magmatic belts and related forearc sediments (the Xilinhot metamorphic complex) to constrain timing of the suturing and to discuss the petrogenesis of the magmatic rocks. A gabbroic diorite (BLD-1) of the Baolidao arc was dated at 310 ± 5 Ma (by SHRIMP). This sample shows an εNd(t) value of +2.5 and ISr of 0.7052. Hf isotopic analyses on 25 zircons from the same sample show εHf(t) = +5.4 to +11.5. Another diorite sample (XH-2) of the same arc from south of Xilinhot displays even more "depleted" isotopic compositions, with εNd(t) = +5.6 and ISr = 0.7037. The main population of zircons from this sample have highly variable and depleted Hf isotopic compositions (εHf(t) = 0-18.3). The large variation in Hf isotopic composition of zircons (with largely the same crystallization age) from a single pluton is explained by a mixing process between depleted mantle-derived magma and continental crust in an active continental arc setting. The Halatu granite (HLT-2) was dated at 234 ± 7 Ma (by SHRIMP). Zircons from the granite also show a large variation of εHf(t) values (+9.1 to -26), despite most samples having whole-rock εHf(t) > +2. The large variation in εHf(t) values suggests that the granite formed probably by partial melting of two source regions - a dominant juvenile crust and a subordinate old continental crust. Most zircons from the Xilinhot metamorphic complex show ages comparable with those of the Baolidao arc rocks, suggesting that the protolith of the metamorphic complex was probably deposited during or after arc magmatism. Some zircons, however, show Precambrian ages that fall into two groups: one with ages of 780-900 Ma, resembling those from the South Mongolian microcontinent, and the other with ages of 1524-2900 Ma, similar to those of the North China Craton. Thus, the protolith of the metamorphic complex probably formed in a forearc basin during convergence of the two continents, and metamorphosed subsequently during collision in the late Paleozoic. Our zircon age data thus constrain timing of collision between the South Mongolian microcontinent and the North China Craton to have been between 296 and 234 Ma. © 2008 Elsevier Ltd. All rights reserved.
SDGs
Type
journal article