Testing the applicability of a partial bleach method for post-IR IRSL dating of Holocene-aged K-feldspar samples
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Thermoluminescence dating
Protolith
Felsic
Banded iron formation
Greenschist
Basement
Geochronology
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The Rae craton on Melville Peninsula, Nunavut, comprises several lithotectonic domains, including a structurally and lithologically distinct yet poorly known crustal terrane, the Repulse Bay block (RBb). This study presents new lithological and petrographic observations, combined with U–Pb zircon data, to better understand the Archean and Paleoproterozoic crustal evolution of the RBb. The new data demonstrate that the central-eastern RBb consists of the following: (i) upper amphibolite- to granulite-facies, ca. 2.73–2.71 Ga intermediate granitoid gneisses and gabbroic sheets; (ii) ca. 2.69 Ga two-pyroxene charnockite to enderbite intrusions; and (iii) thin slivers of both Archean and Paleoproterozoic supracrustal rocks. Inherited zircon also attests to the presence of a Mesoarchean to Paleoarchean substrate. A semi-pelitic gneiss from one of the Paleoproterozoic supracrustal panels was deposited sometime after ca. 1.89 Ga and shows a similar detrital zircon age profile to a <1.92 Ga semi-pelitic gneiss from the Lyon Inlet boundary zone at the northern extent of the RBb. Zircon in most rocks analyzed record metamorphism related to the Trans-Hudson orogeny between ca. 1.84 and 1.82 Ga. Results from this study are most consistent with the RBb, representing a piece of lower to middle crustal level of the Rae craton, rather than a distinct and separate crustal entity (i.e., an exotic block).
Charnockite
Orogeny
Laurentia
Supercontinent
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The Yaogangxian composite granitic pluton is located in the north of Nanling complex structural belt,hosting the giant Yaogangxian tungsten deposit. The Yaogangxian granites are differentiation S-type granite with the characters of highlyacid,hyperalkaline and peraluminous,belonging to the high-K calc-alkalic series granite. LA-ICP-MS zircon U-Pb dating method was applied to obtain the ages of different types of the Yaogangxian granites. The Yaogangxian composite granitic pluton can be divided into three stages according to their emplacement ages,including coarse-grained two-mica granite forming at 170 Ma,medium-to fine-grained two-mica granite forming at 162 Ma,and fine-grained muscovite granite forming at 157 Ma,indicating that the granitic magma had undergone multi-epoch pulsating emplacement. The element geochemistry and Sr-Nd isopotic characters show that source materials of the Yaogangxian granites might come from pelite in the Paleoproterozoic Era. The Yaogangxian granites formed at 170 Ma to 157 Ma belonging to the Yanshannian Period,occurring in a continental crustal extension and thinning setting. The diagenetic events of the Yaogangxian granites and the metallogenetic events of tungsten deposits were highly consistent in time and space. The granitic magma had experienced high fractional crystallization and produced volatile-rich fluid. So it was indicated that the granites may provide original fluid and material source to mineralization in post-magmatic hydrothermal stage.
Fractional crystallization (geology)
Petrogenesis
Pelite
Muscovite
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东秦岭地区是我国重要的花岗伟晶岩区及稀有金属成矿区。电气石在东秦岭各类花岗伟晶岩中广泛发育,通常在无矿化伟晶岩、铍矿化及锂矿化伟晶岩中呈黑色-深蓝色。本文旨在通过各类伟晶岩中电气石的对比研究揭示电气石地球化学特征对东秦岭伟晶岩矿化类型的指示作用。本文所研究电气石为作为东秦岭各类伟晶岩贯通矿物的黑电气石系列。在双峰村、碾盘及风原无矿化伟晶岩中,黑电气石生长环带发育,单偏光下呈蓝绿色-棕色,具有较高的Mg、Ti、Sc含量,较低Li、Mn、Zn含量,δ11B值变化较大(约-19.00‰~-12.00‰)。街子沟富铍伟晶岩中黑电气石单偏光下呈蓝色-草绿色,Ti、V、Sc含量低于无矿化伟晶岩中黑电气石,δ11B值约-16.00‰~-12.00‰。丰庄伟晶岩具弱铌钽矿化,黑电气石具有核-边结构,单偏光下呈蓝色-深蓝色,比无矿化和铍矿化伟晶岩中黑电气石含有更高Li、Zn含量,δ11B值-21.92‰~-14.75‰。在蔡家沟Li矿化伟晶岩中,黑色-深蓝色电气石Li2O含量达到约1.0%,成分上过渡至黑电气石-锂电气石系列,具有最高Li、Mn、Zn含量,而Mg、Ti、Sc、V含量极低,其δ11B值-18.96‰~-16.89‰。硼同位素分析揭示碾盘、风原及丰庄伟晶岩中存在两类电气石,Ⅰ类电气石富重B同位素,可能为较早从伟晶岩熔体中结晶形成;而Ⅱ类电气石具有更负的δ11B值,可能为伟晶岩流体出溶时由流体中晶出。流体出溶导致围岩中形成热液电气石,其δ11B值与伟晶岩中Ⅱ类电气石相似,表明出溶流体与围岩的作用并未导致显著的B同位素分馏。但伟晶岩与围岩之间的作用,使得Mg、Ti、V进入伟晶岩,同时Li、B、Al进入围岩。在研究区内,从无矿化至锂矿化伟晶岩,随伟晶岩分异程度增加,岩浆成因的黑电气石中Li、Mn、Zn、F含量升高而δ11B值降低,表明黑电气石的Li、Mn、Zn及F含量和B同位素组成可以有效指示伟晶岩矿化类型。
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Abundant gold deposits are distributed in the Jiao-Liao-Ji Belt of the North China Craton. Gold deposits, characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock, of the Jiaodong Peninsula and Liaodong Peninsula are underlain by rocks of similar geological settings characterized by uplifted blocks of metamorphosed Archean and Proterozoic rocks surrounded by relatively unmetamorphosed Mesoproterozoic rocks and Mesozoic granitoid plutons. The huge order-of-magnitude difference in reserves between the Liaodong gold province (500 t) and the Jiaodong gold province (5000 t) attract the attention of geologists. Different C–H-O-S-Pb isotopes and isotopic dating have sparked controversy over the source of the gold deposits. In this study, we conducted integrated analyses of zircon U-Pb dating, bulk-rock compositions, mineralogical compositions and high-precision gold analyses on a series of meta-clastic sedimentary rocks from the Paleoproterozoic Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton, of which the Au analyses reveal Au mobilization during prograde metamorphism. In the Gaizhou Barrovian-type metamorphic zone, the bulk-rock Au contents of samples within various metamorphic zones show a monotonic decrease up-temperature, from 2.09 ppb in the chlorite zone to 0.07 ppb in the sillimanite zone. The evolution in Au content during prograde metamorphism of other formations are identical to that of the Gaixian Formation. The Au concentrations of biotite in different metamorphic zones show little variation ranging from 0.66 to 0.35 ppb, whereas the Au concentrations of muscovite in different metamorphic zones mainly decrease from 2.60 ppb in the chlorite zone to 0.57 ppb in the sillimanite zone with increasing metamorphic grade in samples without metallic sulfides. Coincidently, there is a dechlorination process in biotites during regional metamorphism. Chlorine, released by metamorphism, may have played a vital role in the migration of Au, which could absorb onto muscovite and other minerals, during prograde metamorphism. Given an Au content of 4.67 t/km3 in the chlorite zone rock approximately equivalent to the protolith, metamorphism up to upper amphibolite grade would force the rocks to release 52% of the Au in the Gaizhou Barrovian-type metamorphic zone. Taking into account previous studies in the interpretation of our results, we propose that Precambrian rocks, such as the meta-clastic sedimentary rocks of the Gaixian Formation, may have contributed considerable amounts of Au to the Liaodong gold province. The Liaohe Group releasing large amounts of Au during Paleoproterozoic metamorphism, may thus provide the primary Au enrichment for the Mesozoic gold deposits.
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The Niutougou gold deposit is located in the western margin of Ordos block and the northern Helan Mountains. The ore-bearing wall-rocks are a set of medium-deep level metamorphic rocks of the second subgroup of Zongbili Group ( Pt1Z2 ) in Paleo-Proterozoic. Main types of the metamorphic rocks include granulite and biotite-plagioclase-gneiss. Petrochemical study shows that the original rock of the granulite is clastic rock and the original rock of the biotite-plagioclase-gneiss is granodiorite. SIMS zircon U-Pb dating shows that the age of the metasedimentary rock is 2120Ma,biotite-plagioclase-gneiss and diabase are of 1950 ± 8. 9Ma. Authors suggest that the granodiorite should be taken as an intrusive unit apart from metasedimentary system.
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