Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
5779919 | Earth and Planetary Science Letters | 2017 | 13 Pages |
Abstract
Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (â¼540-270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at â¼400 Ma and 280 Ma for the Altai segment, and â¼430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (â¼20-40 Ma), and variations in magma volumes and in Nd-Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting. The Junggar segment is characterized by high and uniform Nd-Hf isotope ratios (εNd(t)=+5 to +8; zircon εHf(t)=+10 to +16) and it appears to have formed in an intra-oceanic arc system. In the Altai and Chinese Tianshan segments, the Nd-Hf isotope ratios (εNd(t)=â7 to +8; zircon εHf(t)=â16 to +16) are lower, although they increase with decreasing age of the rock units. The introduction of a juvenile component into the Chinese Tianshan and Altai granitic rocks appears to have occurred in continental arc settings and it reflects a progressive reduction in the contributions from old continental lower crust and lithospheric mantle. Within the long-lived convergent margin setting (over â¼200 Ma), higher volumes of magma, and greater contributions of juvenile material, were typically emplaced over short time intervals of â¼20-40 Ma. These intervals were associated with higher Nb/La ratios, coupled with lower La/Yb ratios, in both the mafic and granitic rocks, and these episodes of increased magmatism from intraplate-like sources are therefore thought to have been in response to lithospheric extension. The trace element and Nd-Hf isotope data, in combination with estimates of granitic magma volumes, highlight that crust generation rates are strongly non-uniform within long-lived accretionary orogens. The estimated crust generation rates range from â¼0.1 to â¼40 km3/km/Ma for the Paleozoic record of the CAOB, and only comparatively short (20-40 Ma) periods of elevated magmatic activity had rates similar to those for modern intra-oceanic and continental arcs.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Earth and Planetary Sciences (General)
Authors
Gong-Jian Tang, Sun-Lin Chung, Chris J. Hawkesworth, P.A. Cawood, Qiang Wang, Derek A. Wyman, Yi-Gang Xu, Zhen-Hua Zhao,