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Petrogenesis of the Kulongshan Complex Pluton in Northern Hebei:Chronologic and Geochemical Constraints
- GUOJialei，WUJianhua，NIU Ziliang，ZHANG Shaohua，WURengui，LIU Shuai
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2019, 25(1):
33.
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The Kulongshan complex pluton is situated in the northern margin uplift belt of North China Craton and Southwest section of
the Guyuan-Honshanzi uranium belts, which occur between the Chifeng-Kaiyuan fault zone and Shangyi-Pinquan fault zone. The
Kulongshan complex pluton consists of medium-fine alkali-feldspar granite in the middle and medium-coarse alkali-feldspar granite in
the rim, both displaying intrusive contacts and a exposed area of 120 km2. SHRIMP zircon U-Pb dating indicates that the zircon 206Pb/
238U age of medium-coarse alkali-feldspar granite is 129.4±1.5 Ma (MSWD=1.2), and medium-fine alkali-feldspar granite is 134.0±1.7 Ma
(MSWD=1.8). These ages suggest that the granites were formed in the late stage of Early Cretaceous and in the early stage of Early
Cretaceous. Kulonghsan granites show high differentiation features of enrichment in Si, K, Na and high ratios of FeOT/MgO, low
contents of Al, Mg, Ca, P, belonging to the peraluminous-weak high potassium calcium alkaline series rocks. In the SiO2 vs (Na2O +
K2O)-CaO and SiO2 vs FeOT/(FeOT +MgO) diagrams, allthese rocks fall into the Atype granite area. They have low rare earth elements
(REE) contents and display swallow type chondrite-normalized distribution patterns with a distinct negative Eu anomaly. These rocks are
enriched in Rb, Th, U, Nb, Ta, Zr, Hf, Y, and other elements, and are depleted in Sr, Ba, P, Ti, and other elements. These rocks also
display 10000 Ga/Al ratios of 4.63~5.66(＞2.6) and Zr+Nb+Ce+Y values of 402×10-6~713×10-6(＞350×10-6), showing trace element
features of A- type granite. The Kulongshna granites with abnormal initial ratios of Sr: (87Sr/86Sr)i=0.750562~0.839814, relatively low
Nd: εNd(t) =-14.3~-13.2 and young Nd mode age: TDM2=2084~1996 Ma, they exhibit low radiogenic Pb isotopic compositions: (206Pb/
204Pb)t=16.797~17.010, (207Pb/204Pb)t=15.406~15.434, (208Pb/204Pb)t=37.477~37.540. δ18OV-SMOW in medium-coarse alkali-feldspar granites
and medium-fine alkali-feldspar granites were 5.3‰~6.2‰ and 0.1‰~3.1‰, the latter is lower than the normal values of granite (6‰~
10‰) and mantle (5.7‰±0.3‰). The Kulongshan granites have fingerprint of lower crust and enriched mantle in Pb isotope diagrams.
In the tectonic setting discrimination diagrams, the Kulongshan granites belong to the intraplate post orogenic granites, which were
formed in the extensional tectonic setting. Compared with the A type granite of the same age, the Kulongsahn granites were likely the
product of an extensional tectonic system related to the Early Cretaceous and craton destruction in the northern margin of the North
China Craton. This tectonic setting may be related to the subduction of the Pacific Plate. In conclusion, petrogenesis of the Kulongshan
granites can be explained by EM I enriched mantle magma mixed with a small amount of younger crust that were fractionated from
ancient crust material in the Palaeoproterozoic. The genesis of these granites was also likely related to the subducted oceanic slab which
experienced high temperature hydrothermal alteration. However, because the source area was mixed unevenly leading to different
oxygen isotope values of granites, by the Early Cretaceous, the magma formed by partial melting intruded in an intraplate extensional
environment, producing the Kulongshan granites.
