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 (∼540–270 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 ∼400 Ma and 280 Ma for the Altai segment, and ∼430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (∼20–40 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)=−7 to +8; zircon εHf(t)=−16 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 ∼200 Ma), higher volumes of magma, and greater contributions of juvenile material, were typically emplaced over short time intervals of ∼20–40 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 ∼0.1 to ∼40 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.
Highlights
Accretionary orogens form along convergent plate margins and they are major sites of juvenile crust production (Cawood et al, 2013, and references therein)
The topics considered are (i) the implications of crustal thickness and tectonic setting on the observed isotope and trace element changes with time (Figs. 3-7), (ii) the causes of the variations in the volumes of magma emplaced at different times, and (iii) the wider topic of crust generation in accretionary orogens
The age frequency distributions of the mafic and granitic rocks are similar (Fig. 2b), and the isotope and trace element ratios of the mafic rocks can be used to evaluate the nature of mantle derived magmas and how they changed with time (Figs. 4 and 7)
Summary
Accretionary orogens form along convergent plate margins and they are major sites of juvenile crust production (Cawood et al, 2013, and references therein). We present a synthesis of over 2100 whole rock trace element analyses and some 860 Nd isotope and almost 4000 zircon Hf isotope analyses from granitic and mafic rocks in these temporally overlapping accretionary systems These are combined with estimates of the outcrop areas of granitic rocks of different ages in the different segments to evaluate changes in magma volumes, and by implication provide a proxy of magma productivity. The data highlight that most of the magma, and more than 90 % of the juvenile component in the North Xinjiang segments, as evaluated from the surface areas of different granites, was emplaced over short time intervals This has implications for crustal growth in accretionary orogens worldwide
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