气候变化领域集成服务门户(CCPortal): Challenges in constraining the P-T conditions of mafic granulites: An example from the northern Trans-North China Orogen

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DOI	10.1111/jmg.12308
	Challenges in constraining the P-T conditions of mafic granulites: An example from the northern Trans-North China Orogen
	Huang, Guangyu; Brown, Michael; Guo, Jinghui; Piccoli, Philip; Zhang, Dingding
通讯作者	Guo, JH (通讯作者)
发表日期	2018
ISSN	0263-4929
EISSN	1525-1314
起始页码	739
结束页码	768
卷号	36 期号:6
英文摘要	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.
关键词	HIGH-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
英文关键词	high-T metamorphism; mafic granulite; phase equilibrium modelling; Trans-North China Orogen; uncertainty in thermobarometry
语种	英语
WOS研究方向	Geology
WOS类目	Geology
WOS记录号	WOS:000437286400004
来源期刊	JOURNAL OF METAMORPHIC GEOLOGY
来源机构	中国科学院青藏高原研究所
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/259242
推荐引用方式 GB/T 7714	Huang, Guangyu,Brown, Michael,Guo, Jinghui,et al. Challenges in constraining the P-T conditions of mafic granulites: An example from the northern Trans-North China Orogen[J]. 中国科学院青藏高原研究所,2018,36(6).
APA	Huang, Guangyu,Brown, Michael,Guo, Jinghui,Piccoli, Philip,&Zhang, Dingding.(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).
MLA	Huang, Guangyu,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).