华北克拉通北缘隆化地区S型花岗岩的独居石年龄图谱

Monazite age mapping of Longhua S-type granites in the northern margin of the North China Craton.

位于华北克拉通北缘中段的隆化S型花岗岩由石榴石黑云母花岗岩、石榴石花岗岩以及片麻理化的黑云母花岗岩组成。其主体岩性石榴石黑云母花岗岩Si02和A1203含量分别为64．09％～69．6％以及14．6％～16．13％，K20／Na：O〉1．0。A／CNK〉1．0，Mg。在20．76～34．89之间变化，具有明显的Nb、Ta、P、Ti和Sr亏损以及Rb、K和Th富集。石榴石黑云母花岗岩（样品JB6031．1）采用独居石电子探针U-Th-Pb化学法进行测年，获得了2553±120Ma、2180±42Ma和1854±24Ma三个年龄峰值。一颗独居石内部成分分带上6个分析点定年结果构成2553±120Ma的峰值年龄，这一年龄与我们最新获得的2506±7Ma和2541±8Ma（继承锆石年龄）LA-ICP-MS锆石U-Pb同位素年龄相似，我们将这一独居石年龄解释为继承独居石的年龄，表明在赤城．隆化断裂以北存在太古宙陆块，并且在后期构造-热事件中发生部分熔融形成S型花岗岩。该独居石颗粒幔部成分分带上10个分析点的测年结果揭示的峰值年龄为2181±42Ma，该年龄也是出现频率最高的年龄值，我们建议2181±42Ma为S型花岗岩的结晶年龄，反映了S型花岗岩的侵位时代。独居石颗粒外部成分分带上8个分析点的测年结果构成1854±24Ma的峰值年龄，该年龄与华北克拉通中部带的变质年龄接近，我们将其解释为s型花岗岩的变质年龄，表明华北克拉通北缘的构造演化与中部带的构造演化密切相关。

Longhua S-type granites, located in the middle segment of the northern margin of the North China Craton （ NCC ）, consist of garnet biotite granite, garnet granite and gneissic biotite granite. The garnet biotite granite （the major lithology） displays SiO2 concentration of 64.09% -69.6%, A1203 concentration of 14.6% -16.13%, K20/Na20 〉 1.0, A/CNK 〉 1.0, and Mgof 20.76 - 34.89. The trace elements spidergram shows obvious negative anomalies of Nb, Ta, P, Ti and Sr and enrichment of K, Rb and Th. The garnet biotite granite （Sample JB6031-1 ） is dated using EMPA U-Th-Pb dating of monazite, and the monazite ages display three peaks of 2553 ＋ 120Ma, 2180＋42Ma and1854 ＋24Ma. The 6 analytical spots at the inner zone of a monazite grain yield a peak age of 2553 ± 120Ma which is similar to the LA-ICP-MS zircon U-Pb isotopic age of 2506±7Ma and 2541±8Ma （inherited zircon ages we recently acquired）, so we interpret this age as the age of an inherited monazite, revealing that an Archean block exists to the north of Chicheng-Longhua Fault in the northern margin of the NCC and the S-type granites could be derived from partial melting of the lithological assemblage of the Archean block during the late tectonic-thermal event. The 10 analytical spots at the main mantle zone of the monazite grain yield the second peak age of 2180 ± 42Ma, which is the most modal of analyses, so that we suggest that the peak age define crystallization age of the granites, reflecting the displacement times of these granites. The 8 analytical spots at the outer zone of the monazite grain yield a peak age of 1854 ± 24Ma, which is analogical to the metamorphic ages in the Trans-North China Belt, so that we define the age of 1854 ± 24Ma as metamorphic age of the granites, revealing that the tectonic evolution of the northern margin of the NCC is closely related to the Trans-North China Belt.