华北陆台中部孔兹岩原岩年龄及其对基底构造的制约研究

The metamorphic basement of the North China Craton has traditionally been considered as a few volcano-sedimentary stratigraphic sequences. Since lacking geochronological data in the past, to ascertain the age of the sequences was mainly based on metamorphic degree: the mid-high grade rocks was assigned to be an Archean basement and the low grade one was thought to be a Paleoproterozoic volcano-sediment in a rift on the basement. When the Wutai Group consisting of greenschist was supposed to be a Palaeoproterozoic volcanics in rift, the underlying Fuping complex metamorphosed at high amphibolite facies was assigned as the Archean strata in basement. When the Wutai Group was revised to be an Archaean greenston belt, the Fuping complex was undoubtedly thought to be the Archean basement. Therefore, the granulite,gneiss and khondalite in the high-grade region of the central North China Craton has been regarded as Archean rocks or even older..According to our investigation, however, the khondalite is not the Archean supracrustal rocks but a series of Paleoproterozoic sediments in passive margin [Wu Changhua et.al., 1989, 1994, 1997, 1998a], and the Fuping complex contains khondalite in it as well [Wu Changhua et al., 2000]. If so, the traditional ideas that the North China Craton is a uniform Archean basement are not tenable. Age of the khondalite constrains the tectonic model for the Craton. Although the Paleoproterozoic age of the khondalite was obtained through TIMS single zircon U-Pb, but some geologists with Archean standpoint still doubt about it. So SHRIMP or LP-ICPMS method must be taken to confirm our result..As we know, dating the protolith age of unmetamorpic sedimentary rock is not an easy job, let alone mid-high grade khondalite. It was observed that some zircons from khondalite have core-rim texture representing ditrital and overgrowth zircon, respectively. If the ages of the core and the rim of a zircon are obtained, the protolith age will hence be restricted between the two ages. Since only the age of the youngest among all the ditrital zircons can restict the sedimetary time for khondalite, a large number of zircons have to be dated. For this reason, we have to do this mainly by ICPMS, because of the great cost for SHRIMP..441 zircons from 11 khondalites and 5 related rocks from Wulashan, Huangtuyao, Fuping and Jiehekou were dated by ICPMS after taking cathodolumineescence (CL) images. The results reveal that the ages of the khondalites are 1801-1894Ma for metamorophic zircons and 2052-2300Ma for most ditrital zircons except one of 2620Ma, which is from the sample (No. FP 037) of Fuping khondalite in Tanzikou village, Pingshan County. This sample was then tested by SHRIMP, confirming that the ages are 1.8Ga for overgrowth zircon and 2.5 Ga for the oldest ditrital zircon. Hence verified the sedimentation time is after 2.5Ga, i.e. Paleoproterozoic..The fact that the protolith age of the khondalite is not Archean but Paleoproterozoic will shake the foundation of the model that the North China Craton is a uniform Archean basement. It also supports our model that late Paleoproterozoic SW-NE collision resulted in the final assembly of the North China Craton [Wu Changhua et al., 1998b]. In addition, Paleoproterozoic khondalite indicated that the Wulashan and Fuping Groups are not a uniform Archean strata but a shaffle of tectonic slices of different time. Therefore the division of strata in the Craton basement must be reviewed..

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