| Late Neoproterozoic-early Paleozoic was an important period of change in Earth’s evolution,during which global geological events occurred,such as Snowball Earth,the Cambrian Life Explosion,and the transition from the breakup of Rodinia to the assembly of Gondwana supercontinent.Therefore,the process and mechanism of these changes are significant to the reconstruction of the paleoenvironment,paleogeography,paleontological evolution and global deposits during this period.As an important part of the Gondwanaland,the South China Block is a key element to reconstruct the supercontinent’s paleogeographic framework.In comparison with the rare late Neoproterozoic-early Paleozoic magmatic and metamorphic records,clastic sedimentary rocks are relatively well-preserved and widespread in the Cathaysia Block of the South China Block,thus play a key role in provenance analysis,rebuilding basin-mountain coupling and reconstruction for the Gondwana.Previous researches are performed mostly based on detrital zircon provenance analysis of one single sedimentary basin in the Cathaysia block.As a result,several different models have been proposed,which have obfuscated the understanding of the evolution of the Cathaysia Block in the supercontinent cycle.In order to resolve the issues mentioned above,Cryogenian-Ordovician clastic sedimentary rocks of the Cathaysia Block from the Changting region of Fujian Province,Xunwu region of Jiangxi Province,Nanxiong region of Guangdong Province and Hezhou region of Guangxi Province were analyzed.Based on previously published data,field investigation and paleocurrent orientation measurement were carried out.In addition,sedimentological,petrographical,geochemical,detrital zircon geochronological and Hf isotope,and detrital apatite and rutile geochronological and trace element geochemical analyses were carried out.Combined with previous researches on provenance analysis of the Cathaysia Block,the provenance and its variations have been unraveled,together with the paleoposition and evolution of the South China Block in the Gondwana supercontinent assembly.Furthermore,by comparing the data of multiple heavy mineral provenance analyses,their application in the study of Precambrian basins is demonstrated.The main interpretations are as follows:(1)Late Neoproterozoic-early Paleozoic sedimentary rocks in the Cathaysia Block reveal three extensive sedimentary events.Abundance of siliceous and conglomeratic rocks,local unconformities and pinch-out in certain layers indicate a late Cryogenian proximal deposition,late Ediacaran transgression and mid-Cambrian uplift.The uplift,rather than a tectonic quiescence of the southeastern Cathaysia Block during the middle Cambrian,was related to the far-field stress effects of the tectonic movement in western Australia.(2)Detrital zircons indicate that the sedimentary rocks were mainly derived from western Australia and northern India,together with East Antarctica and the Cathaysia Block.Australia and India are the main sources of early(1300-1000 Ma)and late Grenvillian(1000-900 Ma)zircons,respectively,while the detritus from East Antarctica accounts for a lower proportion.India was a prolonged source terrane of the Cathaysia Block,while western Australia changed from a major to secondary source during the late Ediacaran.The middle Cambrian uplift of the Cathaysia Block exposed the intrinsic800-700 Ma magmatic rocks as one of the provenances.The amount of Pan-African(650-500 Ma)detrital zircons is insignificant,which were mainly from India.(3)Detrital apatite and rutile were mainly derived from metamorphic rocks in the Pan-African orogenic belts in western Australia and India.Detrital apatite shares the same 650-500 Ma major age populations with rutile.Apatite data demonstrate a significant amount of detritus contributed from metamorphic(52-72%)and mafic rocks(40-70% on igneous grains),while metabasic rocks account for an appreciable proportion(22-42%)of source rocks of rutile.Three events of provenance variation indicate uplift,exposure and exhumation of crystalline basements in the Pan-African orogenic belts,including unmetamorphosed Grenvillian igneous rocks in the late Cryogenian,high-grade metamorphic rocks in the Cambrian,and old igneous rocks in the early Ordovician.Rutile provenance variations reflect the uplift and exposure of mafic rocks in the Ediacaran.(4)Detrital apatite can record the most sedimentary events.In combination with the variations of apatite source rocks and apatite and zircon age data,the evolution of the source orogens could be outlined in detail.Detrital apatite and rutile have major Pan-African populations while zircon has distinct major Grenvillian populations,reflecting the extensive Pan-African metamorphism in the source orogens.The trace elements of rutile and apatite reflect the same trend of the proportion of mafic materials in the source rocks,confirming the input of sediments from mafic rocks.Moreover,the zircon Lu-Hf isotopic data also support the highest proportion of sediments from mafic and mantle/oceanic crust components in the Ediacaran,which are related to the uplift and denudation in the source orogen.The variations of detrital apatite age and source rock could be the most sensitive indicator of provenance changes,especially for the sediments from metamorphic and basic rocks.Due to the high resistance to post-depositional modification,a large zircon dataset could be readily obtained.On basis of the provenance analyses above,the Cathaysia Block was inferred to locate in the northern margin of Eastern Gondwana during the late Neoproterozoic-early Paleozoic and had persistent connections with India and Australia,adjacent to East Antarctica.During the Gondwana assembly,the formation,uplift and exhumation of the Pan-African orogenic belts in India and Australia influenced the provenance changes of the sedimentary basins in the Cathaysia Block and caused the local uplift of the Cathaysia Block in the middle Cambrian. |
PDF Full Text Request