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摘要
In the southern zone of the Paleoproterozoic Jiao-Liao-Ji Belt (JLJB), North China Craton (NCC), abundant granitic leucosomes are widespread within migmatites as irregular layers, lenses, and blocks. They are parallel to or as cross-cut foliations in the host rocks on various scales, and show distinct evidence of migmatization in the formation of these centimeter- to decimeter-scale segregations. The migmatites extend at least 1100 km in length, spreading over the whole JLJB, from the Ji'an Group in south Jilin, through the South Liaohe Group in Liaodong Peninsula, to the Jingshan Group in Jiaodong Peninsula, rather than as sporadic outcrops limited to the Jiaobei Terrane as reported in previous studies. The granitic leucosomes are comprised mainly of quartz, K-feldspar, and antiperthitic plagioclase (albite), with secondary minerals of garnet, biotite, zircon, etc. They are enriched in SiO2 (74.65-75.54 wt%), Al2O3 (13.25-14.87 wt%), and K2O + Na2O (8.19-8.95 wt%), but have a lower TiO2 + Fe2O3 + FeO + MgO + MnO content (0.42-1.34 wt%). The major element components are similar to those of granitic leucosomes of pelitic granulites in typical orogenic belts around the world. A combined study of whole-rock geochemistry, mineral inclusions, cathodoluminescence (CL) images, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dates of zircons revealed the nature and timing of partial melting of the host pelitic granulites. All dated anatectic zircons from various granitic leucosomes display magmatic zones with distinct cores and rims, and contain mineral inclusions of Qtz + Kfs + Ab +/- Ap in both domains. Both the cores and rims exhibit unusually high U and Th contents, and anomalously low Th/U ratios (< 0.10), indicative of an anatectic origin. The anatectic zircon cores and rims from eight granitic leucosome samples in the Ji'an and Liaohe groups exhibit two distinct weighted mean age groups ranging from 1870 +/- 8 to 1865 +/- 9 Ma and 1843 +/- 13 to 1835 +/- 11 Ma, with age peaks at ca. 1868 +/- 3 Ma and 1841 +/- 4 Ma, respectively. Similarly, abundant anatectic zircon cores and rims from granitic leucosomes in the Jingshan Group from the Jiaobei Terrane also showed two distinct groups with age peaks at 1862 +/- 2 Ma and 1843 +/- 3 Ma, respectively. These two distinct age groups can be interpreted as the initial timing of partial melting and crystallization end age of melts for the migmatites in the whole JLJB. The southern zone of the JLJB experienced a complicated granulite fades metamorphic evolution characterized by clockwise P-T-t paths, rather than anticlockwise P-T-t paths as reported in previous studies. The post-peak MP-LP granulite facies retrogression with a near isothermal decompression P-T path occurred at 1870-1840 Ma, which is completely consistent with the ages of partial melting. These age data provide powerful evidence that the widespread partial melting event within the southern zone of the JLJB occurred at the post-peak near-isothermal decompression stage, rather than at the peak granulite facies stage or the late amphibolite facies cooling stage. Thus, decompression melting is a major control factor on the generation of partial melting, and formation of the granitic leucosomes and migmatites within the southern zone of the JLJB. Under water-unsaturated conditions, hydrous minerals such as biotite underwent dehydration melting, and water released from the biotite results in felsic minerals occurring partial melting. These lines of evidence suggest that the pelitic granulites in the JLJB underwent intensive partial melting during the MP-LP granulite facies "hot" exhumation stage related to the extension and thinning tectonic setting. The resultant melts did not escape from the host rocks, leading to regionally extensive migmatization and subsequent retrogression.
- 出版日期2019-6-15
- 单位中国地质科学院
