Depositional Time, Source Area, And Tectonic Setting Of The Shilu Fe Ore Despoit In Hainan Province Of South China

The Shilu Fe ore deposit, located at the Changjiang County, Hainan Province of South China, is well known for high-grade, hematite-rich Fe ores, with subordinate Co, Cu, Ni, Pb-Zn and Ag resources. The ore bodies are hosted in two Precambrian host successions, i.e. the Shilu Group and the overlying Shihuiding Formation. The Shilu deposit has been interpreted as a banded iron formation(BIF) deposit-type, but its depositional time, source area and depositional setting have been in debate due to poor geochronological work, which has seriously constrained the next prospecting on the depth and margin of minning area. Based on the previous researches, detrital zircon U-Pb dating aided by cathodoluminescence imaging has been carried out on both the Shilu Group and Shihuiding Formation, in order to pinpoint the maximum depositional time, source area and depositional setting of the studied successions, and thereby re-evaluate Precambrian tectonic evolution history of Hainan Island.The Shihuiding Formation is better re-interpreted as the top, i.e. Seventh member of the Shilu Group, rather than a distinct Formation. Both the two successions have age spectra between 2000 Ma and 900 Ma with two predominant peaks at ca. 1460-1340 Ma and 1070 Ma, and three subordinate peaks at ca. 1740-1660 Ma, 1220 Ma and 970 Ma. The similar age distribution suggests the same depositional system for both the successions. This is consistent with the recognition that the Shihuiding Formation contains a similar megafossil assemblage to the Shilu Group. Thin-section observation also reveals that the Shihuiding Formation had the same metamorphic grade of greenschist-facies which is defined by the characteristic mineral assemblage and texture as the Shilu Group. Furthermore, the fault contact relationship for both successions has been interpreted as a result of the responses to late-stage tectonic activities due to the rheological competency contrast between rock types from both the Shihuiding Formation and the Sixth member of the Shilu Group. Thus, it is reasonable to reinterpret the Shihuiding Formation as the top, i.e. the Seventh member of the Shilu Group.The initial Shilu Group and interbedded BIFs were formed primarily during 1070-970 Ma, and last to 880 Ma. The youngest statistical detrital zircon age peaks for both successions, i.e. ca. 1070-970 Ma, may define the main depositional time of the Shilu Group(i.e. the First to Sixth member of the Shilu Group). Combined with chemical U-Th-total-Pb(CHIME) dating results on monazite(880 Ma), depositional time of the Shilu Group is suggested to last to 880 Ma.Detrital zircon ages from the Shilu Group and interbedded BIFs are similar to that observed in Laurentia and Australia, especially the Laurentia, implying the possible continents such as Australia and Laurentia as the potential provenance during the assembly of Rodinia. Given that the Shilu Group has typical Grenville-aged detrital zircons and Hainan Island was still attached to Laurentia during or shortly after the Grenvillian orogeny, it is possible that a marine, retro-arc foreland basin received detritus from an unknown, uplifted orogen comprising a continental magmatic arc and older basement and sediments for deposition of the host rocks and interbedded BIFs in Shilu district. Such depositional setting is consistent with the features of typical foreland basins, and also supported by the geochemical affinities of back-arc basalts for the metamorphosed mafic volcanic rocks from the Baoban Group, which in return suggest the presence of a late Paleoproterozoic to Mesoproterozoic arc system before the deposition of the Shilu Group.In comparison with those from the South China and other typical Grenvillian orogens, the detrital zircon age populations reveal that Hainan Island had crystalline basement similar to neither the Yangtze nor the Cathaysia Blocks. Combined with the tectonothermal events, we propose that Hainan Island was independent of South China at least before the late Ordovician and most likely attached or close to northwestern Laurentia before the breakup of Rodinia. The detrital zircon population with typical Grenvillian ages(ca. 1300-1000 Ma) for the Shilu Group suggests that Hainan Island was derived from Grenville orogen(s) neither in the Yangtze and Cathaysia Blocks nor in the Eastern Ghat and East Antarctica, but adjacent to western Laurentia and central Australia(1300-1050 Ma) or to one part of Laurentia. Linked to the lack of the Neoproterozoic-early Paleozoic granites and the Neoproterozoic glaciogenic sediments, as well as the presence of the Mesoproterozoic granites, the present study implies that Hainan Island likely was independent of South China at least before the late Ordovician.The results of this paper support the view that the Shilu deposit is a banded iron formation(BIF) deposit-type, which had been re-enriched by multistage and multiple structural deformations, metamorphism and hydrothermal alteration. The Shilu Group and interbedded BIFs were deposited in a restricted marine, retro-arc foreland basin received detritus from Australia and Laurentia during the assembly of Rodinia(1070-970 Ma), and the re-enrichment of ores was related to multistage and multiple structural deformations, metamorphism and hydrothermal alteration.