| Southeastern (SE) China has long been widely accepted to be controlled by paleo-Pacific tectonic regime during Late Mesozoic, the plate subudction model however remains controversial. There are several questions yet to be resolved including:when the subduction initiated, migration of magmatic zones was caused by changes of subduction direction or slab rollback, what the subduction polarity was, the slab rollback did exist or not and how to identify it. It will make a big difference by determining the exact migration pattern of the magmatic zones and the role played by crust-mantle interaction. Late Mesozoic volcanic rocks in SE China are widely distributed and continuously formed with plenty standard stratigraphic sections, thus they are more representative both spatially and temporally but less concerned than their intrusive counterparts. This study for the first time presents the systematic zircon U-Pb ages and Lu-Hf isotopic features of the Late Mesozoic volcanic rocks in SE China, to determine their spatial-temporal framework and influences and changes of crust-mantle interaction in their petrogenesis. The results offer new perspectives and contribute to better understanding the paleo-Pacific subudction mechanism and Late Mesozoic tectono-magmatic evolution in SE China.Maonong Formation, the earliest Late Mesozoic volcanic sequence in SE China, was formed at ca.177 Ma. The lower and upper volcanic series in Zhejiang were comparably formed in four episodes of volcanism:140-130,130-127,123-118 and 110-88 Ma respectively, among which the second episode made the volcanism climax and the last one took place only in southeastern Zhejiang. The volcanic sequences in different areas comprises of similar rock assemblages of pyroclastics-rhyolite-bimodal volcanic rocks from bottom to top, although each type of rock was generated earlier inland than in coastal area. Hence, the most widespread volcanic sequences in SE China simply show an oceanward (southeastward) younging trend. The upper volcanic series in Fujian was formed also during 110-88 Ma, in a good agreement with that in Zhejiang. A series of volcanic sequences in Chiyuan, Fujian which was proposed to be formed during early Jurassic has been determined with ages of 160-148 Ma and treated as part of late Jurassic Changlin Formation. Nanyuan Formation formed during 145-130 Ma represents the product during volcanism climax in Fujian, and the latest and small scale Xiaoxi Formation with ages of 130-127 Ma correspond temporally to the product of volcanism climax in Zhejiang. Thus there also shows a northeastward younging trend from eastern Fujian to southeastern Zhejiang for the lower volcanic series in coastal SE China.All of he studied volcanic rocks in each area show zircon Hf-isotopic features from isotopic enrichment to depletion with increasing εHf(t) values over time, except for Changlin Formation which shows opposite variation trend. The most isotopic enriched volcanic rocks display Paleoproterozoic crustal model ages, implying that they were mainly formed by reworking of the crustal basement materials. Basaltic magmas derived from depleted mantle underplated beneath lower crust, and contributed as both a heat source inducing crustal melting and a source of materials that mixed with the local crustal magmas in increasing proportions over time. Moreover, it should be mentioned that magma mixing started since different times in different areas based on the large dataset. The magma mixing took place gradually from inland to coastal area within Zhejiang and from eastern Fujian to southeastern Zhejiang in coastal SE China, just as well as the migration (younging) trend of volcanic sequences.Late Mesozoic volcanic rocks in SE China are distributed mainly in coastal area which is obviously controlled by large faults within crust, indicating that the ancient subduction zone was primarily parallel to the present coastline. Note this and that the volcanic sequences in Zhejiang simply migrated oceanward, it suggests that the paleo-Pacific plate subducted northwestward beneath continental lithosphere of SE China during Late Mesozoic. The underplated basaltic magmas were mainly derived from wet partial melting of mantle wedge which closely connected to dehydration of the subducted slab. Since slab dehydration can only occur at certain deep range, the oceanward migration of volcanic sequences in Zhejiang implies an increasing dip angle of subducted slab. On the other hand, the pyroclastics-rhyolite-bimodal volcanic rock assemblages and increasing juvenile contribution in the volcanic petrogenesis both suggest enhanced lithospheric extensional setting, which also gradually passed from inland to coastal area. The setting would be a robust response caused by southeastward slab rollback. Nonetheless, magma mixing gradually took place northeastward in coastal area during 145-118 Ma, indicating asynchronizing slab rollback in different latitude. The typical bimodal volcanic rocks of upper series suggest back-arc extensional tectonic setting, further implying a high dip angle of the entire subducted slab.The Changlin Formation is distributed to the east part of Nanling Range, where has been suggested to be under within-plate extensional tectonic setting and lots of ca.160 Ma granitic bodies were formed. Large extent of crust-mantle interaction induced by crustal extension is considered to play an important role in their petrogenesis, which may also be how the lower Changlin Formation volcanic rocks of 160 Ma age were formed. Should juvenile proportion in magma mixing represent the stress condition of the continental lithosphere, the change from isotopic depletion to enrichment of Changlin Formation volcanic rocks would suggest enhanced compressional setting, as a good response to forward subduction soon after subduction initiation. It also suggests that the subducted initiated a little earlier than 160 Ma, probably at early-middle Jurassic. |
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