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Challenges in constraining the P-T conditions of mafic granulites: An example from the northern Trans-North China Orogen
Huang, GY (Huang, Guangyu)1,2,3; Brown, M (Brown, Michael)3; Guo, JH (Guo, Jinghui)1; Piccoli, P (Piccoli, Philip)3; Zhang, DD (Zhang, Dingding)4
Source PublicationJOURNAL OF METAMORPHIC GEOLOGY
2018-08-01
Volume36Issue:6Pages:739-768
DOI10.1111/jmg.12308
Abstract

Some mafic granulites in the Sanggan area of the northern Trans-North China Orogen (TNCO) have a relatively simple mineralogy with low energy grain shapes that are compatible with an assumption of equilibrium, but the rock-forming minerals show variations in composition that create challenges for thermobarometry. The mafic granulites, which occur as apparently disrupted dyke-like bodies in tonalite-trondhjemite-granodiorite gneisses, are divided into two types based on petrography and chemical composition. Type 1 mafic granulites are fine- to medium-grained with an equilibrated texture and an assemblage of plagioclase+clinopyroxene+garnet+magnetite+ilmenite and sometimes minor hornblende +/- orthopyroxene. Type 2 mafic granulites are coarse-grained and hornblende bearing with a peak assemblage of garnet+clinopyroxene+plagioclase+hornblende and variably developed coronae and symplectites of plagioclase+hornblende+orthopyroxene partially replacing porphyroblastic garnet +/- clinopyroxene. SIMS U-Pb dating of metamorphic zircon from two type 1 mafic granulites yields metamorphic ages of c. 1.84 and 1.83Ga, consistent with published ages of the type 2 mafic granulites. Based on phase equilibrium modelling, we use the common overlap of P-T fields defined by the mineral assemblage limits, and the mole proportion and composition isopleths of different minerals in each sample to quantify the metamorphic conditions. For type 1 granulites, overlap of the mineral proportion and composition fields for each of three samples yields similar P-T conditions of 710-880 degrees C at 0.57-0.79GPa, 820-850 degrees C at 0.59-0.63GPa and 800-860 degrees C at 0.59-0.68GPa. For the type 2 granulites, overlaying the peak assemblage fields for three samples yields common P-T conditions of 870-890 degrees C at 1.1-1.2GPa. For the retrograde assemblage, overlap of the mineral proportion and composition fields for each sample yields similar P-T conditions of 820-840 degrees C at 0.85-0.88GPa, 860-880 degrees C at 0.83-0.86GPa and 880-930 degrees C at 0.89-0.95GPa. The P-T conditions appear distinct between the two types of mafic granulite, with the mineralogically simple type 1 mafic granulites recording the lowest pressures. However, there are significant uncertainties associated with these results. For the granulites, there are uncertainties related to the determination of modes and composition of the equilibration volume, particularly estimation of O and H2O contents, and in the phase equilibrium modelling there are uncertainties that propagate through the calculation of mole proportions and mineral compositions. The compound uncertainties on pressure and temperature for high-T granulites are large and the results of our study show that it may be unwise to rely on P-T conditions determined from the simple intersection of calculated mineral composition isopleths alone. Since the samples in this study are from a limited areaa few hundred square metreswe infer that they record a single P-T path involving both decompression and cooling. However, there is no evidence of the high-P granulite facies event at 1.93-1.90Ga that is recorded elsewhere in the TNCO, which suggests that the precursor basic dykes were emplaced late during the assembly of the North China Craton.

Subject Area地质学
WOS IDWOS:000437286400004
Language英语
Indexed BySCIE
KeywordHigh-pressure Granulites 1000-degrees-c Metamorphic Temperatures Paleoproterozoic Tectonic Evolution Mineral Equilibria Calculations Phase-equilibria Bulk Composition Basement Rocks Crustal Growth Enderby Land Lithotectonic Elements
WOS Research AreaGeology
WOS SubjectGeology
Cooperation Status国际
ISSN0263-4929
Department大陆碰撞与高原隆升重点实验室
PublisherWILEY
Citation statistics
Cited Times:6[WOS]   [WOS Record]     [Related Records in WOS]
Document Type期刊论文
Identifierhttp://ir.itpcas.ac.cn/handle/131C11/8586
Collection图书馆
Corresponding AuthorBrown, M (Brown, Michael); Guo, JH (Guo, Jinghui)
Affiliation1.Chinese Acad Sci, Inst Geol & Geophys, State Key Lab Lithospher Evolut, Beijing, Peoples R China;
2.Univ Chinese Acad Sci, Beijing, Peoples R China;
3.Univ Maryland, Dept Geol, Lab Crustal Petrol, College Pk, MD 20742 USA;
4.Chinese Acad Sci, Inst Tibetan Plateau Res, Beijing, Peoples R China.
Recommended Citation
GB/T 7714
Huang, GY ,Brown, M ,Guo, JH ,et al. Challenges in constraining the P-T conditions of mafic granulites: An example from the northern Trans-North China Orogen[J]. JOURNAL OF METAMORPHIC GEOLOGY,2018,36(6):739-768.
APA Huang, GY ,Brown, M ,Guo, JH ,Piccoli, P ,&Zhang, DD .(2018).Challenges in constraining the P-T conditions of mafic granulites: An example from the northern Trans-North China Orogen.JOURNAL OF METAMORPHIC GEOLOGY,36(6),739-768.
MLA Huang, GY ,et al."Challenges in constraining the P-T conditions of mafic granulites: An example from the northern Trans-North China Orogen".JOURNAL OF METAMORPHIC GEOLOGY 36.6(2018):739-768.
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