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Serpentinites associated with metamafic and metaplagiogranitic rocks occur sporadically within metasedimentary schists in the Yuli belt. Such metaigneous rocks are considered to represent ophiolitic protoliths and some contain high-pressure (HP) metamorphic minerals or assemblages. However, the origin and metamorphism of these serpentinites have long been a mystery. This study systematically investigates the four major serpentinite-bearing exposures at the Fengtien, Wanjung, Tsunkuanshan, and Chinshuichi. The results reveal that the serpentinites are of two different protoliths on the basis of relict chromian spinel composition. Cr-spinel compositions of the Fengtien and Tsunkuanshan samples are characterised by moderate Cr# [Cr/(Cr + Al)] (0.45–0.57), relatively high Mg# [Mg/(Mg+Fe2+)] (0.59–0.79), but low Fe3+# [Fe3+/(Fe3++Cr+Al)] (0.02–0.06), reflecting an abyssal peridotite type protolith. By contrast, Cr-spinel compositions of the Wanjung and Chinshuichi samples show high Cr# (up to 0.74) and Fe3+# (0.03–0.08), but low Mg# (<0.6), indicating a forearc mantle peridotite origin. Peak metamorphic conditions of the serpentinites are represented by the assemblage of antigorite + magnetite + chlorite + olivine + diopside, which is estimated as up to 550°C but the pressure cannot be constrained quantitatively. Based on field relations, it is inferred that the serpentinites, associated HP metaigneous rocks, and garnet-bearing metasedimentary schists were metamorphosed isofacially. Despite two origins, the peridotitic protoliths were all subjected to hydration and subduction processes. We suggest that precursors of the serpentinites, metaigneous rocks, and metasedimentary schists were juxtaposed and metamorphosed at intermediate to great depths (~35–55 km) of a subduction zone. Therefore, the Yuli belt serpentinites and associated rocks likely represent exhumed materials from a palaeo-subduction interface. [ABSTRACT FROM AUTHOR]

The largest gabbroic intrusion, Jiaoziyan, in the Northern Orthogneiss of the Dabie Shan was investigated to test whether such intrusions represent arc-related or collision-related magma, as postulated in recent regional tectonic models. The Jiaoziyan intrusions comprise a gabbro-monzogabbro suite and are locally characterized by cumulus textured anhydrous silicates. Petrologic indications against arc magmatism include (1) coexisting olivine-plagioclase compositions beyond the documented range of arc gabbros, and (2) a compositional trend of Al/Ti ratios in the gabbroic clinopyroxenes that is significantly different from subduction-related gabbros worldwide. In contrast, the Jiaoziyan rocks are mineralogically similar to rift gabbros. Geochronologic and geometric arguments against collision-related magmatism (i.e., as a consequence of slab breakoff) include the following: (1) crystallization of the Jiaoziyan mafic intrusion postdated the Triassic (245-240 Ma) subduction and peak orogenic metamorphism by ~100 m.y. and (2) the rocks of the Dabie Cretaceous magmatic suite, including Jiaoziyan, are on the footwall plate, rather than on the hanging wall, of the Triassic subduction-zone framework and do not show a linear distribution. These observations argue that the voluminous Early Cretaceous magmatism in the Northern Orthogneiss is geodynamically unrelated to the Qinling-Dabie orogeny, but was caused by extensional tectonism related to regional magmatic under-plating beneath the thinned Dabie crust. Keywords: gabbros, petrogenesis, extension tectonics, Cretaceous, Dabie Mountains.

Mineral assemblages and microstructures of a newly identified (retrogressed) eclogite (sensu lato) in the North Dabie Complex (NDC), central-eastern China, indicate early very high-pressure metamorphic relics preserved in a dominant amphibolite-facies host, where no eclogite (sensu stricto) has been positively identified before. The investigated eclogitic rock shows distinct multistage recrystallization, with granulite- and amphibolite-facies assemblages overprinting eclogite-facies relics. The minimum temperature for the eclogite-facies metamorphism is estimated to be ~800-820 [degrees] C. A spectacular microstructure of oriented quartz needles (~2-20 [micro]m wide, ~5-200 [micro]m long) in matrix Ca-Na clinopyroxene implies the prior existence of a non-stoichiometric 'supersilicic' omphacite stabilized at ultrahigh-pressure (UHP, [is greater than or equal to] 25 kbar) conditions, although no coesite or coesite pseudomorphs have been found in the samples. The absence of coesite may be due to the lack of free silica at UHP conditions or the consumption of silica during retrograde reactions. The inferred UHP conditions metamorphism is further supported by Sm-Nd ages that are equivalent to Triassic metamorphic ages from UHP eclogites in the southeastern Dabie Mountains. This finding expands the UHP terrane northward about 50 km; spatial distribution of subduction/collision-related UHP rocks includes parts of the NDC.

It remains unclear whether a crystalline basement exists in SE China (including Taiwan), whether the formation of the Tananao metamorphic belt in Taiwan was linked to subduction of the Palaeo-Pacific Plate, and whether the source rocks of the sedimentary sequences in the metamorphic belts are late Mesozoic or Palaeozoic in age. Field investigations and zircon age data in the present study indicate that there is no pre-Palaeozoic gneiss (crystalline basement) in Taiwan (although orthogneisses were produced during deformation and metamorphism of Mesozoic granites), and investigations of the metasediments show that the sedimentary sequences in the Tailuko and Yuli belts are similar. Moreover, LA-ICP-MS dating of detrital zircons from the Pingtan–Dongshan belt in Fujian Province yields a cluster of 206Pb–238U ages at ~ 210–190 Ma, and the Tailuko and Yuli belts in Taiwan have similar clusters of detrital zircon ages at 200 Ma, 160 Ma, 120 Ma and 110 Ma, as well as a later overprinting caused by arc–continent collision. The cathodoluminescence images and trace-element characteristics of the zircons show that they were originally magmatic in origin. This finding, combined with the Hf isotope data, indicates that the sources of sediments in the Tananao belt (Tailuko and Yuli belts) were relatively close to an active continental margin, and that both the Tailuko and Yuli belts have similar sedimentary sources. From the margin of the Chinese mainland to Taiwan, the metasediments seem to represent a continuous sequence of deposits ranging in age from Jurassic to Cretaceous, but with the sediments becoming progressively younger towards the east. It can be inferred that the sediments in the Tailuko and Yuli belts were continental-shelf sequences with sources in SE China. [ABSTRACT FROM AUTHOR]