Sr–Nd–Pb isotope mapping of Mesozoic igneous rocks in NE China: Constraints on tectonic framework and Phanerozoic crustal growth

Abstract

New Sr–Nd–Pb isotope dataset of Mesozoic igneous rocks shows that the NE China can be subdivided into four Sr–Nd–Pb isotope provinces: (1) the northern Hinggan Mountains (NHM) region; (2) the southern Hinggan Mountain (SHM)–Yanji-Liaoyuan (YL) region; (3) the Zhangguangcai Range–Jiamusi (ZGJ) Block; and (4) the exotic Wandashan massif (WDM). The Wandashan massif contains Mesozoic (high-μ)-type oceanic island basalts (HIMU-OIBs) with highly radiogenic Pb isotopic compositions (e.g., 206Pb/ 204Pb(i) = 18.9–22.7), quite different from other regions that have moderately radiogenic Pb (i.e., 206Pb/ 204Pb(i)) is generally less than 18.6. These HIMU-OIBs also show negative Δ7/4 and Δ8/4 values, signatures of Southern Gondwanaland Continent. By contrast, the majority of Mesozoic igneous rocks in other areas of NE China have positive Δ7/4 and Δ8/4 values, akin to the Northern Laurasian Continent. Such isotopic variations were probably due to the enrichment processes mainly caused by Paleozoic Paleo-Asian ocean subduction and to some extent by subduction of the Paleo-Pacific Ocean since early Mesozoic. The mantle-derived rocks in the NHM region is characterized by moderately radiogenic Nd and Pb isotopic compositions (ε Nd(t) = −0.2 to + 3.6 and 206Pb/ 204Pb(i) = 18.3–18.6). Compared with the NHM region, the mantle-derived magmas in the SHM–YL region show wider Nd isotopic variation (ε Nd(t) = −1.1 to + 6.6) and less radiogenic Pb ( 206Pb/ 204Pb(i) = 18.1–18.4). Along the Hegenshan-Solonker suture distributes an early Cretaceous felsic magmatic belt with highly positive-ε Nd (ε Nd(t) = + 4.0 to + 5.9) and radiogenic Pb compositions ( 206Pb/ 204Pb(i) = 18.4–18.6), suggesting that it was not only an important Phanerozoic crustal growth belt but also a zone containing significant volume of pelagic sediments or their metamorphosed derivates. Adjacent to the Hengenshan ophiolite occurs a suite of andesites showing nonradiogenic Nd (ε Nd(t) = −10.5 to −6.3) and Pb ( 206Pb/ 204Pb(i) = 17.22–17.26), as well as DUPAL Pb isotopic signatures. These features indicate the existence of “old” lower continental crust beneath the area. Compared with the majority of crust-derived rocks in the NHM and SHM–YL regions (ε Nd(t) = −2.9 to + 6.8 and 206Pb/ 204Pb(i) = 17.4 to 18.6), the Mesozoic granitoids in the ZGJ block have relatively less radiogenic Nd (ε Nd(t) = −2.9 to + 2.1) and more radiogenic Pb composition ( 206Pb/ 204Pb(i) = 18.2 to 18.9). Considering that the Paleozoic granitoids have nonradiogenic Nd (ε Nd(t) < −6), we suggest that pelagic sediments or their metamorphosed derivates were an important crustal component in the region. The new isotope mapping results suggest that during Mesozoic time the crustal growth mainly occurred around the collisional sutures (e.g., the Hegenshan-Solonker suture) and/or along the major lithosphere-scale faults (e.g., the Tan-Lu fault), in which highly positive-ε Nd rocks are distributed. However, the extent of Mesozoic crustal growth was subordinate relative to the Paleozoic time in terms of Nd isotopic evolution. Generation of Mesozoic voluminous felsic magmas in NE China was mainly ascribed to remelting, recycling and redistribution of the preexistent crustal components (juvenile and “old” crustal components), which had been juxtaposed during the tectonic evolution of the Paleo-Asian Ocean.