Detrital Zircon Geochronology and Provenance of Metasedimentary Rocks from the Susong Complex Zone in the Dabie Orogen

The Susong complex zone is a relatively low-grade metamorphic unit located in the southern part of the Dabie orogen and preserves a variety of metasedimentary rocks,mostly with epidote-amphibolite facies.However,their depositional age,provenance and tectonic setting of sedimentary protolith remain controversial due to the lack of fossils,precise dating and integrated geochemical investigation.This study has conducted whole-rock elemental,and zircon U-Pb SHRIMP dating and Lu-Hf isotope analyses on three types of representative metasedimentary rocks including garnetbearing mica-quartz schist,graphite-muscovite-quartz schist and dolomitic marble from the Susong complex zone.The UPb SHRIMP dating data indicate that the sedimentary protolith of these rocks have the maximum depositional ages of less than 840-750 Ma and have various sedimentary provenances.In combination with zircon Lu-Hf isotope compositions,the provenance of the metasedimentary rocks in the region is for the first time documented to be mainly derived from four groups of magmatic rocks formed at~2.5 Ga,~2.0 Ga,~1.4 Ga and~0.8 Ga in response to four episodes of igneous activity in the northeastern margin of the Yangtze Block.The conventional geothermobarometry combined with rare metamorphic zircon ages indicate that the studied rocks underwent the Late Triassic continental subduction-related metamorphism with peak epidote-amphibolite facies conditions at P=0.34-0.91 GPa and T=427-532°C.In addition,results of the elemental contents(i.e.,La,Ce,Th andΣREE)and ratios(i.e.,Eu/Eu^(*)and La_(N)/Yb_(N))suggest that the protoliths of the metasedimentary rocks were mainly shales,wackes and limestones,most probably related to the Rodinia supercontinent rifting along the northern margin of the Yangtze Block during the Neoproterozoic.

Geochemical characterization of the metasedimentary rocks of the Yaounde Group within the southernmost North Equatorial tectonic belt:insights into geodynamic evolution

The Proterozoic metasedimentary rocks of the Yaounde Group on the northern edge of the Congo Shield in Central Africa were investigated to understand their provenance and depositional environment.Petrography,geochemistry,and field evidence helped to subdivide the metasediments into paragneiss,mica schist,chlorite schist,and quartzite which were derived from greywacke,shale,quartz arenite,litharenite protoliths.They are immature with some mature samples,moderately weathered and reworked Neo-and Post-Archean metasediments.Rare earth element signatures(Chondrite Eu/Eu^(*)≤1),enrichment of light rare earth elements over the heavy ones,and the La/Sc ratio(>0.7)are compatible with those of the intermediate and felsic sources from the upper continental crust.These metasediments were deposited in the continental arc setting and have evolved during Proterozoic times according to the Wilson cycle to form the West Gondwana including NE Brazil.

Geochemistry and petrogenetic evolution of metasedimentary rocks in Bunu Area,part of Kabba-Lokoja-Igarra schist belt,SW Nigeria

Field and geochemical studies of the Bunu area,SW Nigeria,were carried out on gneiss-metasedimentary rocks that consist of migmatised gneiss,quartz-mica schist,and foliated and massive quartzites.These gneissmetasedimentary rocks are interbedded with meta-igneous rock(amphibolite),all of which are intruded by granitoid and cut by basic and felsic dykes.Geochemical data on major,trace,and rare Earth elements of these rocks from the area were used to speculate on the petrogenetic and geodynamic evolution of the rocks in the area.Compositionally,the metasedimentary rocks in the area have a restricted range of major oxides such as SiO_(2)and Al_(2)O_(3)with low average values of FeO,MnO,CaO,and PO.Concentrations of average values of HSFE such as Zr,Nb,and Y are moderately high in migmatitic gneiss,quartzmica schist,and low in both massive and foliated quartzite.ΣLREE average values are 124.25,132.41,and18.64 ppm respectively for migmatite gneiss,quartz-mica schist,and low in both massive and foliated quartzite.These rocks are also generally enriched in Ba,Cs,Pb,U,Cr,Rb,and Zr and depleted in Be,Sr,Ti,Mo,Th,and W.This enrichment–depletion in major oxides and trace elements of the rocks in this area is probably a consequence of the movement of metamorphic remobilized fluids within the rocks in the area during the Pan-African or earlier events as noted in the adjacent Egbe-Isanlu Schist belt.A further geochemical characterisation of the rocks in the area using Log(Na_(2)O/KO)vs Log(SiO_(2)/Al_(2)O_(3)indicates that while the protolith of migmatised gneiss and quartzmica schist are greywackes to litharenite in compositions that of quartzites are sublitharenite to quartz arenite.On the ternary plot of Al_(2)O_(3)-(CaO+Na_(2)O)-KO for metasedimentary rocks in the area,most migmatitic gneiss and quartz-mica schist rocks plot close to average shale while quartzites mostly tend to illite compositions and almost toward the Al_(2)O_(3)-apex of the diagram attesting to depletion of CaO and Na_(2)O with the removal of KO.CIA vs PIA plot of the rocks in the area shows that while migmatitic-gneiss and quartz-mica schist are moderately weathered,both massive and foliated quartzites are high to extremely weathered.They are all derived mostly from predominantly felsic igneous to quartzose sedimentary provenance and mostly emplaced in passive continental margins.These nature of the protoliths suggests gradual subsidence of the basin during its genesis,and/or tectonic stable or inactive environment from which the sediments were derived.

Magnetic Fabric of Late Devonian Metasedimentary Rocks from Longyan,Southwestern Fujian,China

This study presents the results of magnetic fabric on Late Devonian metasedimentary rocks from Longyan , Southwestern Fujian , China .Measurements of low - field anisotropy of magnetic susceptibility (AMS)were made on 59 specimens (sites 48-53 ) using a Sapphire SI-2 induction coil instrument. Our data show that the magnetic fabric at sites 48- 51 and 53 is mainly carried by paramagnetic minerals (muscovite.chlorite and biotite), while at site 52 both paramagnetic minerals and hematite contribute to the total AMS . Separation of the two components of anisotropy indicates that they are approximately coaxial . The magnetic foliation is either parallel to an Indosinian (Late Triassic ) metamorphic schistosity (sites 48-52 ) or to a Yanshanian (Late Mesozoic ) microscopic mylonitic foliation (site 53). The magnetic lineation corresponds either to me regional structural trend (sites 48 - 52 ) or to the line of intersection between the schistosiry and the mylonitic surface (site 53 ).Our results suggest that Indosinian metamorphism and deformation took place simultaneously under the control of a compressional stress field which is mostly responsible for the generation of the observed magnetic fabric .However ,post-Indosinian deformation of the Yanshanian orogeny may exist at some locations and interfere with the Indosinian fabric .

Paleoproterozoic,High-Metamorphic,Metasedimentary Units of Siberian Craton

Sensitive, high-resolution ion microprobe zircon U-Pb ages of Paleoproterozoic, high-grade, metasedimentary rocks from the south-western part of the Siberian Craton are reported. Early Precambrian, high-grade complexes, including garnet-biotite, hypersthene-biotite, and cordierite- bearing gneisses compose the Irkut terrane of the Sharyzhalgay Uplift. Protoliths of studied gneisses correspond to terrigenous sediments, ranging from greywacke to shale. The paragneiss model Nd ages of 2.4-3.1 Ga indicate Archean-to-Paleoproterozoic source provinces. Zircons from gneisses show corerim textures in cathodoluminescence （CL） image. Round or irregular shaped cores indicate detrital origin. Structureless rims with low Th/U are metamorphic in origin. The three age groups of detrital cores are： 〉2.7, -2.3, and 1.95-2 Ga. The ages of metamorphic rims range from 1.86 to 1.85 Ga; therefore, the sediments were deposited between 1.95 and 1.86 Ga and derived from Archean and Paleoproterozoic source rocks. It should be noted that Paleoproterozoic metasedimentary rocks of the Irkut Block are not unique. High-grade metaterrigenous sediments, with model Nd ages ranging from 2.3 to 2.5 Ga, are widely distributed within the Aldan and Anabar Shields of the Siberian Craton. The same situation is observed in the North China Craton, where metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 to 2.1 Ga （Wan et al., 2006）. All of these sedimentary units were subjected to Late Paleoproterozoic metamorphism. In the Siberian Craton, the Paleoproterozoic sedimentary deposits are possibly marked passive margins of the Early Precambrian crustal blocks, and their high-grade metamorphism was related to the consolidation of the Siberian Craton.

Nature and(in-)coherent metamorphic evolution of subducted continental crust in the Neoproterozoic accretionary collage of SW Mongolia

In continental subduction complexes minor volumes of high-pressure mafic rocks(eclogites)often co-exist with much more abundant felsic(granitic)and metasedimentary rocks,which are vital for resolving the origin and metamorphic evolution of subducted continental crust.In SW Mongolia,the Alag Khadny eclogite-bearing accretionary complex(AKC)is assumed to represent either a remnant of oceanic slab,or a continental margin,subducted in the Early Cambrian.Here we present geochronological,geochemical and petrological evidence of subduction records for the three major types of lithologies that host mafic eclogites,including Mesoproterozoic and Neoproterozoic granitic basement and overlying Neoproterozoic continental-margin sediments.Variably deformed,ferroan and peraluminous metagranitoids compose a major part of AKC and are interlayered with eclogites in its southern and eastern margins.They have geochemical features of post-collisional/intraplate high-K calc-alkaline granites.LA-ICP-MS U-Pb zircon geochronology of three distinct metagranite samples show uniform protolith crystallization ages of ca.0.96 Ga and uncertain re-crystallization in the Late Neoproterozoic or Early Paleozoic metamorphic event,whereas abundant zircon inheritance indicates older,Mesoproterozoic to Paleoproterozoic crustal substrate during granite generation.The existence of Mesoproterozoic crust is highlighted by finding of distinct metagranitoids with the U-Pb zircon crystallization age of ca.1.6 Ga.Hafnium isotope signatures(T_(DM)^(C)2.88-1.85 Ga)of zircons from all lithologies preserved the evidence of reworked Neoarchean to Paleoproterozoic crust,similar to that of the Baidrag block(southern Mongolia),for both Mesoproterozoic and Neoproterozoic rocks.Regardless of the specific lithology,the rocks display indicators of high-pressure metamorphic re-equilibration,including garnet(X_(Ca)up to 0.65)+epidote+phengite(Si p.f.u.up to 3.56)±rutile assemblage in metagranitoids,garnet+phengite(Si p.f.u.up to 3.42)in quartz-rich semi-pelites and garnet+phengite(Si p.f.u.up to 3.39)+medium-Mg chloritoid(X_(Mg)up to 0.25)+kyanite+rutile in metapelites.Corresponding P-T conditions recovered from different lithologies reveal incoherent subduction of rocks,which could be shallow for granitic basement(1.1-1.4 GPa and 600-670℃)and clastic metasediments(1.4-1.6 GPa,570-620℃),but deeper for metapelites(2.1-2.3 GPa,500-570℃).consistent with that of eclogites,The combined data show that the Alag Khadny complex represents a remnant of a rifted Mesoproterozoic to Neoproterozoic(ca.1.6-0.96 Ga)continental margin consequently metamorphosed under HP conditions during Late Neoproterozoic-Early Cambrian evolution of the southern Central Asian Orogenic Belt.Acquired P-T estimates imply that high-pressure metagranitoids and metasedimentary rocks equilibrated at different depths,but most likely shared a common subduction-related metamorphic evolution.

Paleozoic tectonic evolution of the proto-Korean Peninsula along the East Asian continental margin from detrital zircon U-Pb geochronology and Hf isotope geochemistry

Detrital zircon geochronology and Hf isotope analysis can be used for inferring provenance characteristics,and to evaluate the tectonic evolution of sedimentary basins and their link with regional orogenesis.The Paleozoic sequences of the Okcheon Belt consist of the Lower Paleozoic Joseon and the Upper Paleozoic Pyeongan supergroups with Middle Paleozoic hiatus locally on top of the Neoproterozoic bimodal volcanic rocks,reflecting an intracontinental rift setting between the two basements(viz.Gyeonggi and Yeongnam massifs)at southern part of the Korean Peninsula.Our detrital zircon U-Pb ages and Lu-Hf isotope results show that all these Paleozoic strata commonly have Paleoproterozoic and Paleozoic zircon ages with rare Meso-to Neoproterozoic ages.The individual zircon populations display following features,allowing estimation of their sedimentary provenances:(i)The Paleoproterozoic zircons(ca.1.85 Ga and 2.50 Ga)with similar ranges ofεHf(t)values are most common in the basement rocks of the Korean Peninsula,and were sourced from both the Gyeonggi and Yeongnam massifs.(ii)The Meso-to Neoproterozoic zircons,preserved only in the Middle to Late Cambrian clastic sedimentary rocks within the carbonate sequences probably reflect proximal provenance.(iii)The youngest Paleozoic zircons of each formation,almost coincident with their deposition ages,suggest presence of syndepositional magmatism,indicating proximal magmatic sources during their deposition.(iv)The Cambrian-Ordovician zircons,from the Lower Paleozoic sequences,but rare in the successive Upper Paleozoic sequences,suggest a provenance change after the hiatus between the two sedimentary successions.(v)The Permian zircons showing differentεHf(t)values indicate that detrital sources were varied at that time.The integrated results in our study suggest provenance variability linked to diverse tectonic environments,reflecting prolonged subduction-related crustal evolution of the proto-Korean Peninsula during the Paleozoic.