Rhythmic Layering in the Apatite (-Magnetite) Deposit in the Fanshan Complex, North China |
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Authors: | Chun Cheng Shihua Sun Qian Mao |
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Institution: | Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China [e-mail: ];Registered additionally in the Graduate School of Chinese Academy of Sciences, Beijing 100029, China Received on October 17, 2003;accepted on June 9, 2004 |
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Abstract: | Abstract. The Fanshan complex consists of three (First- to Third-) Phases of intrusion. The First-Phase Intrusive is composed of rhythmic layers. The Fanshan apatite (-magnetite) deposit is situated near the top of no. 6 rhythmic Unit of the First-Phase Intrusive. Two sketch maps were drawn through the deposit along no. 25 cross-cut on 425 mL and no. 1 crosscut on 491 mL in the Fanshan mine. Through this mapping, a small-scaled rhythmic layering (called sub-rhythmic layering) was newly found. The sub-rhythmic layering consists of five typical rocks: biotite clinopyroxenite, biotite rock, biotite-apatite rock, alternation biotite-apatite rock and apatite rock. The thickness of layers varies from four millimeters to several meters. Six units of sub-rhythmic layering were distinguished. Each unit consists of biotite clinopyroxenite (or biotite rock and biotite-apatite rock) layer at the bottom and apatite rock layer at the top. Major element compositions of biotite and clinopyroxene were analyzed. The trends of Mg/(Mg+Fe*+Mn) atomic ratio (Fe*: total iron) of biotite and clinopyroxene indicate that the magma evolved markedly from relatively magnesian bottom layer to less magnesian top layer in each sub-rhythmic unit. A general trend, however, through the whole sub-rhythmic layering sequence shows that the both minerals become relatively magnesian upwards. The hypotheses of liquid immiscibili-ty, gravitational settling and magma mixing are against with the results obtained in the present study. This study suggests that the apatite (-magnetite) deposit is formed through fractional crystallization process. |
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Keywords: | Fanshan apatite (-magnetite) deposit sub-rhythmic layering fractional crystallization Mg/(Mg+Fe*+Mn) atomic ratio biotite clinopyroxene |
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