The petrochemistry of the Lower Proterozoic metamorphic rocks from the Dabieshan-Lianyungang area, the southeastern margin of the North China Platform

Abstract

Based on geological studies, 141 rock analyses and 5 trace element analyses of metabasites, the present paper deals with the rock association, chemical features, protolith formation and the original tectonic settings upwards through the Lower Proterozoic metamorphic strata in the Dabieshan-Lianyungang area, in the south-east of the North China Platform. The results of the study indicate that the lower and middle parts of the metamorphic strata comprise terrigenous clastics, phosphoritic and aluminous sedimentary formations which formed under stable continental margin conditions. In the middle-upper part a calc-alkaline volcano-sedimentary formation under the active continental margin was developed. The Lower Proterozoic meta-strata of sedimentary-volcanosedimentary origin from bottom upwards suggest that the tectonic evolution of the south-eastern margin of the North China Platform is a process from stabilization to mobilization. This process suggests a northward subduction of the Yangtze Plate under the North China Plate during the later part of the early Proterozoic.

A refined late-Cryogenian – Ediacaran earth history of South China: Phosphorous-rich marbles of the Dabie and Sulu orogens

“…Our observations showed, like those made by previous workers (e.g. Longkang 1991), that the pebbly schist is marked by a fine-grained micaceous-quartzitic matrix containing numerous, widely dispersed clasts of gneissic rocks, quartzites and rare fine-grained (meta)sedimentary clasts, all from 1 cm to more than 50 cm in size. In other words, it is a metamorphosed diamictite.…”

“…As noted above, the Haizhou Group was originally inferred to be Palaeo-Mesoproterozoic in age and correlated with rocks in the Dabie Orogen that were thought to be of similar age (Longkang 1991;Yang et al 1986; see discussions by Xu et al 2012 andZhou et al 2012). U-Pb dating on detrital zircons recovered from the Haizhou Group, however, confirm that these units can be no older than Ediacaran (Fig.…”

“…It is at least several kilometres thick and garnetiferous amphibolites are indicative of upper greenschist to lower amphibolite facies metamorphism, consistent with the documentation of the adjacent blueschist facies rocks of the Sulu orogen (phengite, glaucophane and jadeite with P-T estimates of 0.7-0.85 GPa and 350º-450° C) that were retrogressed to upper greenschist facies during the late Triassic (Qiu et al 2003;Wang et al 2017;Xu et al 2006). Whilst mapping the eponymous type area of the Hiazhou Group, Longkang (1991) identified four units ( Fig. 3A): (i) migmatised granitoid and felsic orthogneiss; (ii) a discontinuously developed pebbly schist from 1-10 m in thickness that overlies unconformably the gneissic rocks; (iii) a unit of marble, P-rich marble and graphiticpelitic schist; and (iv) interlayered quartz-albitic and amphibolitic schists.…”

“…Both features contain phosphorous-rich (P-rich) rocks; those in the former occur as phosphorites and have been studied extensively (e.g. She et al 2013;Muscente et al 2015;Cui et al 2016) whereas those in the latter occur as apatite-rich units and are far less understood, as evident by the uncertainty in assigning their depositional age with estimates initially ranging from Palaeo-to Mesoproterozoic and now considered to be late Neoproterozoic (Longkang 1991;Pirajno 2013;Yang et al 1986; see discussion by Xu et al 2012).…”

“…B. Distribution of successions containing P-rich rocks adjacent to the Tan-Lu fault zone (from Longkang 1991). C. Simplified rock-distribution map of the Haizhou-Jinping area with location of U-Pb detrital zircon ages obtained by Zhou et al (2012).…”