Chemical U-Th-Pb Monazite Dating of Deformations versus Pluton Emplacement and the Proterozoic History of the Arkansas River Region,Colorado,USA

Five lengthy periods involving multiple phases of cordierite and andalusite growth were revealed by detailed studies of foliation inflection/intersection axes （FIA） preserved in porphyroblasts in schists from the Arkansas River region in Colorado, USA. The regionally consistent character of the succession of five different FIA trends enabled the relative timing of each FIA with respect to the next to be determined. The FIA succession from first to last is： FIA 1 trending W-E, FIA 2 trending SSW- NNE, FIA 3 trending NNW-SSE, FIA 4 trending NW-SE and FIA 5 trending SW-NE. For four of the FIA sets, samples were found containing monazite grains preserved as inclusions. These were dated on an electron microprobe. The ages obtained concur exactly with the FIA succession, with FIA 1 at 1506±15 Ma, FIA 2 at 1467±23 Ma, FIA 3 at 1425±18 Ma, FIA 4 not dated and FIA 5 at 1366±20 Ma. These ages are directly reflected in a succession of plutons in the surrounding region dated by other isotopic approaches, suggesting that deformation, metamorphism and pluton emplacement occurred together episodically, but effectively continuously, for some 140 Ma.

U-Th-Pb monazite and Sm-Nd dating of high-grade rocks from the Grove Mountains, East Antarctica: further evidence for a Pan-African-aged monometamorphic terrane

The Grove Mountains, 400 km south of the Chinese Antarctic Zhongshan Station, are an inland continuation of the Pan-African-aged （i.e., Late Neoproterozoic/Cambrian） Prydz Belt, East Antarctica. In this paper we carried out a combined U-Th-Pb monazite and Sm-Nd mineral-whole-rock dating on para- and orthogneisses from bedrock in the Grove Mountains. U-Th-Pb monazite dating of a cordierite-bearing pelitic paragneiss yields ages of 523 ？ 4 Ma for the cores and 508 ？ 6 Ma for the rims. Sm-Nd mineral-whole-rock isotopic analyses yield isochron ages of 536 ？ 3 Ma for a coarse-grained felsic orthogneiss and 507 ？ 30 Ma for a fine-grained quartzofeldspathic paragneiss. Combined with previously published age data in the Grove Mountains and adjacent areas, the older age of ~530 Ma is interpreted as the time of regional medium- to low-pressure granulite-facies metamorphism, and the younger age of ~510 Ma as the cooling age of the granulite terrane. The absence of evidence for a Grenville-aged （i.e., Late Mesoproterozoic/Early Neoproterozoic） metamorphic event indicates that the Grove Mountains have experienced only a single metamorphic cycle, i.e., Pan-African-aged, which distinguishes them from other polymetamorphic terranes in the Prydz Belt. This will provide important constraints on the controversial nature of the Prydz Belt.

Structures of Syn-deformational Granites in the Longquanguan Shear Zone and Their Monazite Electronic Microprobe Dating

The Longquanguan shear zone is an important structural belt in the North China Craton, separating the underlying Fuping complex from the overlying Wutai complex. This shear zone has experienced three episodes of deformation: the first and main episode is a ductile top-to-ESE shear along the gently northwest-west dipping foliations, while the other two episodes are later collapse sliding. Prolonged granites parallel to the shear foliations make one of the main compositions of the Longquanguan shear zone. These granites experienced deformation to form mylonitic rocks when they emplaced during the first episode of deformation. Structural characters of the granites and their contacts to the country rocks indicate that these granites possibly resulted from in-situ partial remelting by shearing, i.e., they are syn-deformational granites. Monazites in these mylonitic granites are magmatic minerals and their crystallization ages may represent ages of the magmatic events, and also the ages for the main deformation of the Longquanguan shear zone. Monazite electronic microprobe dating were carried on two samples of granite, which gives multiple peak ages, among which 1,846 Ma and 1,877 Ma are the main peak ages for the two samples. These ages represent the main deformation of the Longquanguan shear zone, which is consistent with the main regional geological event at about 1,850 Ma caused by the collision between the Eastern and Western Blocks in North China. The good match between the monazite ages and the corresponding regional tectono-thermal events shows the feasibility and reliability of monazite electronic microprobe dating.

Monazite U-Th-Pb and sericite Rb-Sr dating of the Xiajiadian black shale-hosted gold deposit in the Qinling orogen,central China:Implications for regional gold metallogeny

The Qinling orogen in central China contains several black shale-hosted gold deposits,but the age of their formation and a possible relation between gold mineralization and regional tectonism remain undetermined.Here we present results of in situ monazite U-Th-Pb dating and sericite Rb-Sr dating of the Xiajiadian black shale-hosted gold deposit to provide tight constraints on the time of gold deposition and information on the tectonic setting under which the deposit formed.The Xiajiadian gold deposit is mainly hosted in black shales of the Lower Cambrian Shuigoukou formation,with minor ores contained in sandy conglomerate of the Lower Devonian Xichahe formation.Gold ores in the black shales have been intensively oxidized and consist mainly of hematite,goethite,gypsum,quartz,calcite,native gold,and unrecognized iron oxides.However,gold ores in the sandy conglomerate are free of oxidation and thus provide an opportunity to date the formation of the Xiajiadian gold deposit.The ores consist of pyrite,marcasite,sericite,and quartz,which are associated with a variety of accessory minerals,including monazite,apatite,and titanite.Gold is mainly present as structure-bound ions or nanoparticles in pyrite and marcasite,but minor native gold grains also occur as inclusions in pyrite.Monazite is well developed in the ores,and has close textural association with Au-bearing pyrite and marcasite.Most monazite grains have zoned textures consisting of a core area(Mnz1)and an overgrowth zone(Mnz2).Mnz1 contains 4.8–13 wt.%ThO_(2)and shows large variations in HREE and prominent negative Eu anomalies in the chondrite-normalized REE patterns.It yields common lead-corrected^(208)Pb/^(232)Th dates ranging from 910±10 to416±4 Ma(2σ).These dates are interpreted in terms of a detrital origin for monazite in the core area.Mnz2 has much lower ThO_(2)contents of 0.8–1.7 wt.%and shows weak negative Eu anomalies in the chondrite-normalized REE patterns.It yields common lead-corrected^(208)Pb/^(232)Th dates of 229–209 Ma(2σ)with a weighted mean of 217.6±3.0 Ma(2σ),which is considered as the time of Mnz2 crystallization.Sericite in gold ores and associated alteration zones has close relations to Au-bearing pyrite and marcasite,and yields a well-defined Rb-Sr isochron age of 222.2±3.3 Ma(2σ).The sericite Rb-Sr age is indistinguishable within analytical errors from the weighted mean Th-Pb age of Mnz2.The age data presented here indicate that the Xiajiadian gold deposit formed at ca.220 Ma in the Late Triassic(Norian).We therefore suggest that gold mineralization at Xiajiadian occurred during the transitional stage from oceanic subduction to continental collision that led to the formation of the Qinling orogen.During this stage,metamorphic devolatization of the underlying Early Paleozoic to Proterozoic carbonaceous sequences likely supplied ore fluids from which the Xiajiadian gold deposit formed.

Crystal Chemistry and Geochronology of Thorium-Rich Monazite from Kovela Granitic Complex,Southern Finland

Abundant porphyritic granites, including Grt-bearing and Bt-bearing porphyritic granites, and porphyritic potash-feldspar granite (trondhjemite-granitic composition) are widely distributed within the Kovela granitic complex Southern Finland, which associated with monazite-bearing dikes (strong trondhjemite composition). The investigated monazite-bearing dikes are dominated by a quartz + K-feldspar + plagioclase + biotite + garnet + monazite assemblage. The monazite forms complexly zoned subhedral to euhedral crystals variable in size (100 - 1500 μm in diameter) characterized by high Th content. The chemical zoning characterised as: 1) concentric, 2) patchy, and 3) intergrowth-like. Textural evidence suggests that these accessory minerals crystallized at an early magmatic stage, as they are commonly associated with clusters of the observed variations in their chemical composition are largely explained by the huttonite exchange , and subordinately by the cheralite exchange with proportions of huttonite (ThSiO4) and cheralite [CaTh(PO4)2] up to 20.4% and 9.8%, respectively. Textural evidence suggests that these monazites and associated Th-rich minerals (huttonite/thorite) crystallized at an early magmatic stage, rather than metamorphic origin. The total lanthanide and actinide contents in monazite and host dikes are strongly correlated. Mineral compositions applied to calculate P-T crystallization conditions using different approaches reveal a temperature range of 700°C - 820°C and pressure 3 - 6 kbars for the garnet-biotite geothermometry. P-T pseudo-section analyses calculated using THERMOCALC software for the bulk compositions of suitable rock types, constrain the PT conditions of garnet growth equilibration within the range of 5 - 6 kbars and 760°C - 770°C respectively. Empirical calculations and pseudo-section approaches indicate a clockwise P-T path for the rocks of the studied area. 207Pb/206Pb dating of monazite by LA-MC-ICPMS revealed a recrystallization period at around 1860 - 1840 Ma. These ages are related to the tectonic-thermal event associated with the intense crustal melting and intra-orogenic intrusions, constraining the youngest time limit for metamorphic processes in the Kovela granitic complex.

Geochemical and geochronological characteristics of the Um Rus granite intrusion and associated gold deposit,Eastern Desert,Egypt

The Um Rus tonalite-granodiorite intrusion(~6 km2)occurs at the eastern end of the Neoproterozoic,ENE-trending Wadi Muba rak shear belt in the Central Eastern Desert of Egypt.Gold-bearing quartz veins hosted by the Um Rus intrusion were mined intermittently,and initially by the ancient Egyptians and until the early 1900 s.The relationship between the gold mineralization,host intrusion,and regional structures has always been unclear.We present new geochemical and geochronological data that help to define the tectonic environment and age of the Um Rus intrusion.In addition,field studies are integrated with EPMA and LA-ICP-MS data for gold-associated sulfides to better understand the formation and distribution of gold mineralization.The bulk-rock geochemical data of fresh host rocks indicate a calc-alkaline,metaluminous to mildly peraluminous,I-type granite signature.Their trace element composition reflects a tectonic setting intermediate between subduction-related and within-plate environments,presumably transitional between syn-and post-collisional stages.The crystallization age of the Um Rus intrusion was determined by in situ SHRIMP 206 Pb/238 U and 207Pb/235U measurements on accessory monazite grains.The resultant monazite U-Pb weighted mean age(643±9 Ma;MSWD 1.8)roughly overlaps existing geochronological data for similar granitic intrusions that are confined to major shear systems and are locally associated with gold mineralization in the Central Eastrn Desert(e.g.,Fawakhir and Hangaliya).This age is also consistent with magmatism recognized as concomitant to transpressional tectonics(D2:~650 Ma)during the evolution of the Wadi Mubark belt.Formation of the gold-bearing quartz veins in NNE-SSW and N-S striking fault segments was likely linked to the change from transpressional to transtensional tectonics and terrane exhumation(D3:620-580 Ma).The development of N-S throughgoing fault arrays and dike swarms(~595 Ma)led to heterogeneous deformation and recrystallization of the mineralized quartz veins.Ore minerals in the auriferous quartz veins include ubiquitous pyrite and arsenopyrite,with less abundant pyrrhotite,chalcopyrite,sphalerite,and galena.Uncommon pentlandite,gersdorffite,and cobaltite inclusions hosted in quartz veins with meladiorite slivers are interpreted as pre-ore sulfide phases.The gold-sulfide paragenesis encompasses an early pyrite-arsenopyrite±loellingite assemblage,a transitional pyrite-arsenopyrite assemblage,and a late pyrrhotite-chalcopyrite-sphalerite±galena assemblage.Free-milling gold/electrum grains(10 sμm-long)are scattered in extensively deformed vein quartz and in and adjacent to sulfide grains.Marcasite,malachite,and nodular goethite are authigenic alteration phases after pyrrhotite,chalcopyrite,and pyrite and arsenopyrite,respectively.A combined ore petrography,EPMA,and LA-ICP-MS study distinguishes morphological and compositional differences in the early and transitional pyrites(PyⅠ,PyⅡ)and arsenopyrite(ApyⅠ,ApyⅡ).Py I forms uncommon small euhedral inclusions in later PyⅡand Apy II.PyⅡforms large subhedral crystals with porous inner zones and massive outer zones,separated by narrow As-rich irregular mantles.The Fe and As contents in PyⅡare variable,and the LA-ICP-MS analysis shows erratic concentrations of Au(<1 to 177 ppm)and other trace elements(e.g.,Ag,Te,and Sb)in the porous inner zones,most likely related to discrete sub-microscopic sulfide inclusions.The outer massive zones have a rather homogenous composition,with consistently lower abundances of base metals and Au(mean 1.28 ppm).The early arsenopyrite(Apy I)forms fine-grained euhedral crystals enriched in Au(mean 17.7 ppm)and many other trace elements(i.e.,Ni,Co,Se,Ag,Sb,Te,Hg,and Bi).On the other hand,ApyⅡoccurs as coarsegrained subhedral crystals with lower and less variable concentrations of Au(mean 4 ppm).Elevated concentrations of Au(max.327 ppm)and other trace elements are measured in fragmented and aggregated pyrite and arsenopyrite grains,whereas the undeformed intact zones of the same grains are poor in all trace elements.The occurrence of gold/electrum as secondary inclusions in deformed pyrite and arsenopyrite crystals indicates that gold introduction was relatively late in the paragenesis.The LAICP-MS results are consistent with gold redistribution by the N-S though-going faults/dikes overprinted the earlier NNW-SSE quartz veins in the southeastern part of the intrusion,where the underground mining is concentrated.Formation of the Um Rus intrusion and gold-bearing quartz veins can be related to the evolution of the Wadi Mubarak shear belt,where the granitic intrusion formed during or just subsequent to D2 and provided dilatation spaces for gold-quartz vein deposition when deformed by D3 structures.

Anticlockwise metamorphic paths at ca.890–790 Ma from the NE Baidrag block,Mongolia,indicate back-arc compression at the Rodinia periphery

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known.In contrast,accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood.To fill this gap,we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt,where numerous data testify for Meso-to Neoproterozoic magmatic reworking.The tectono-metamorphic evolution of the periSiberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian(Baikalian)cycle.However,we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block,previously considered as a typical example of the Baikalian metamorphic belt.This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist.Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at 720℃ and 6.0 kbar followed by further burial to the kyanite stability field at 750℃ and 9 kbar and decompression to 650℃ and 8 kbar.The Grt-St schist records initial burial to the staurolite stability field at 620℃ and 6 kbar,followed by further burial to 590℃ and 8.5 kbar.The monazite data yield a continuum of ^(207)Pb-corrected ^(238)U/^(206)Pb dates of ca.926–768 Ma in the Grt-Sil-Ky gneiss,and ca.937–754 Ma in the Grt-St schist.Based on monazite textural positon,internal zoning,and REE patterns,the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32℃/km is estimated at ca.890–853 Ma.It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22℃/km dated at ca.835–815 Ma.The late monazite recrystallization at ca.790 Ma is related to decompression.Additionally,monazite with dates of ca.568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint.Metamorphic zircon rims with Th/U ratios of 0.01–0.06 in Grt-Sil-Ky gneiss with 877±7 Ma age,together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism.The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice–probably of back-arc or arc type.This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean.The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly,followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.

Metamorphic Evolution of Chloritoid-Bearing Micaschist from the Variscan Elstergebirge: Evidences for Stacking of High-Pressure Rocks in the Saxothuringian Zone of Central Europe

A chloritoid-garnet-bearing micaschist from the southern part of the Elstergebirge was studied to better understand the Variscan orogenic evolution in the Saxothuringian zone of the northwestern Bohemian Massif.Based on the textural relations and compositions of minerals,especially of zoned garnet and potassic white mica,a P-T path was reconstructed using contoured P-T pseudosections.The U-Th-Pb dating of monazite in the micaschist was undertaken with the electron microprobe.The micaschist experienced P-T conditions along a clockwise path between 16 kbar at 510℃and 5 kbar at 555℃followed by isobaric heating to about 600℃.Monazite ages range between 315 and 480 Ma with the most prominent maxima and side maxima at 346.0±1.1(2σ),357.3±2.3,and 368.3±1.7 Ma.Ages older than 380 Ma were related to detrital monazite pointing to a Devonian sedimentary protolith.Other ages around 325 Ma were assigned to the isobaric heating by nearby post-tectonic granites.The high-pressure event,being the result of the collision of Laurussia and Gondwana after closure of the Rheic Ocean,occurred in the Late Devonian.The exhumation to 15-20 km(5 kbar)ended probably in the Early Carboniferous.The high-pressure micaschists from the Fichtelgebirge to the Erzgebirge crystalline complexes are suggested to represent a single nappe within a metamorphic nappe pile.This nappe is composed of metasedimentary slices,which experienced different peak pressures rather than representing a coherent crustal section.