Genetic mechanism of meta-igneous rocks in the Co-bearing Li'eryu Formation,Liaohe Group:Constraints on the tectonic evolution of the Jiao-Liao-Ji Paleoproterozoic orogenic belt
The Li'eryu Formation of the Liaohe Group,including meta-basic rocks and meta-felsic rocks,preserves significant information related to the thermodynamic and geochemical evolution of the Jiao-Liao-Ji orogenic belt.These rocks not only serve as critical stratigraphic units for cobalt enrichment in the Jiao-Liaodong region,but also constitute essential elements for unraveling the Paleoproterozoic tectonic background and evolutionary processes of the Jiao-Liao-Ji orogenic belt.In this study,a comprehensive approach,involving detailed field investigations,petrology,zircon geochronology and Lu-Hf isotope analyses,and whole-rock geochemistry,was employed to thoroughly investigate the protolith characteristics,formation mechanisms,geochronological framework and tectonic background of the meta-basic rocks and meta-felsic rocks in the Li'eryu Formation.Moreover,a preliminary discussion was carried out to establish an intrinsic relationship between the Paleoproterozoic tectonic evolution of the Jiao-Liao-Ji orogenic belt and the initial sources of cobalt.The Li'eryu Formation is primarily composed of meta-basic rocks,mainly consisting of amphibolites and meta-hornblende rocks,while the meta-felsic rocks include leptite and leptynite.Field observations reveal that mafic rocks intruded into the felsic plutonic rocks within the study area.Zircon U-Pb dating results indicate that the protolith formation age of the meta-basic rocks ranges within 2160～2130Ma,while the meta-felsic rocks were formed in 2200～2160Ma.These rocks underwent a Paleoproterozoic metamorphic event during the 1950～1850Ma.Based on the geochemical characteristics,the meta-basic rocks can generally be divided into three types,and each of them has different features as follows:(1)The first type of amphibolites falls within the tholeiitic series and exhibits slight enrichment in LREE,resembling E-MORB features;and they notably display slight depletions in Nb,Ta,Ti,Zr and Hf,suggesting limited fluid alteration and crustal contamination;furthermore,the metamorphic zircon εHf(t)values range from-2.3～5.9,with tDM1 concentrated around 2200～2100Ma,indicating the origin from a depleted asthenospheric mantle accompanied by juvenile crust growth during the Paleoproterozoic.(2)The second type of amphibolites belongs to the calc-alkaline series and displays a right-skewed Rare Earth Element(REE)distribution curve;and they show significant depletions in Nb,Ta and Ti,but exhibit positive Zr and Hf anomalies,indicating that their original magma experienced crustal contamination after leaving its source region.(3)The third type of meta-hornblende rocks falls within the range of ultramafic rock compositions,which exhibit a relatively high total REE content with relative enrichment in LREE,and significant depletions in Nb,Ta,Ti,Zr and Hf,indicating limited crustal contamination and substantial fluid alteration;furthermore,the magmatic zircon εHf(t)values range from-2.3～6.7,with tDM1 concentrated around 2500～2200Ma;while metamorphic zircon εHf(t)values range from-5.9～-0.4,with tDM1 concentrated in the 2500～2300Ma,indicating a source from the enriched lithospheric mantle.Within the meta-felsic rocks,the leptynite exhibit lower SiO2 content compared to the leptite.However,both of the leptynite and leptite belong to the calc-alkaline series and exhibit similar geochemical characteristics,suggesting a similar magmatic source.The meta-felsic rocks are characterized by LREE enrichment and relatively depleted in HREE.They also show positive anomalies in Zr and Hf,but exhibit significant depletions in Nb,Ta and Ti,resembling characteristics commonly associated with arc-related continental crustal rocks.Their magmatic zircon εHf(t)values range from-2 to 4,while metamorphic zircon εHf(t)values range within-7～-1.Both of their tDM2 concentrated around 2900～2600Ma,suggesting the protoliths of these rocks likely originated from the partial melting of the Mesoarchean TTG lower continental crust,indicating a process of continental crust recycling.Both the meta-felsic rocks and the meta-mafic rocks indicate their tectonic setting as an arc-back basin,transitioning through the phases of arc-back rifting to collisional orogeny.The tectonic evolution can be outlined as follows:(1)During the period of 2200～2160Ma,the plate between the Nangrim block and the North Australian Craton subducted from southwest to northeast.Subsequently,the subducting oceanic plate retreats,instigating lithospheric extension and initiating the incipient rifting of the back-arc basin.This extension process led to the upwelling of the asthenosphere,resulting in the heating of the overlying Archean continental crust.Prolonged thermal influence led to partial melting of the continental crust,concomitant with the contemporaneous genesis of a significant volume of acidic volcanic rock and intrusive acidic rocks.(2)During the period of 2160～2130Ma,the subducting oceanic plate continued to retreat on the southern margin of the Nangrim block,and led to further lithospheric extension and thinning.The rifting of the back-arc basin continued during this process and resulted in additional upwelling of the asthenosphere.The consequent decompression melting gave rise to the formation of the 2160～2130Ma basic magma.Simultaneously,the upwelling of the asthenosphere heated the overlying lithospheric mantle,caused partial melting,and produced a limited amount of enriched mafic magmas.(3)During the period of 1950～1850Ma,the ongoing collision between the Nangrim and Longgang blocks initiated the development of the orogeny,with the regional stress regime transitioning from extensional to compressional.This tectonic shift marked the entry of the back-arc basin into an orogenic process.The transition of regional rocks from amphibolite to granulite facies signifies an intense orogenic period within the basin.Furthermore,the source of cobalt may be related to basic magmatism.During volcanic eruptions,fluids along fractures extracted Co from basic rocks,resulting in the formation of cobalt rich layers in the Li'eryu Formation,which underwent the transformation of Paleoproterozoic metamorphic and deformation events.
Jiao-Liao-ji orogenic beltLi'eryu FormationMeta-basic and meta-felsic rocksGeochronologyGeochemistryBack-arc basin
万方数据
维普
