On The Geochemical Characteristics And Genesis Of Clinopyroxene In Cenezoic Basalts From South China

Clinopyroxene is one of the major rock-forming minerals in mafic magma systems,being an important phase including trace elements of basalts and chemically recording valuable information concerned with magma evolution and physicochemical condition.There are two perspectives for clinopyroxene megacryst genesis,the first of which is the capture by ascending basaltic magma after MME crash and the other is that clinopyroxene megacrysts derive from the same source of host magma or clinopyroxeneis the crystallization product of basalt magmas under the high pressure condition.This paper presents the clinopyroxene megacryst genesis,deep geological information,source nature and geology significance by means of the analyses for major and trace elements in samples from seven locations of Bailin(BL),Dayangke(DY),Shiheng(SH)in Fujian province,Xiadai(XD),Caijiawan(CJ)in Zhejiang province,Jiucaidi(JC)in Guangdong province and Fujitian(FJT)in Hainan province.The KDFe-Mg for clinopyroxene megacrysts and phenocrysts ranges from 0.2-0.4 and correlates linearly with the content of major elements in that MgO correlates with Si02 positively,while negatively with CaO,Al2O3 and FeO,respectively.The correlation indicates that clinopyroxene megacrysts may possibly have crystallized from basalt magmas.Obtained temperatures and pressures by clinopyroxene mineralogical themobarometer show:1354?,22.5Kbar(or 72km)for SH;1366-1370?.22.4-23Kbar(or71-73km)for XD;1299-1303??17.2-17.8 Kbar(or56-57km)for JC;Therefore,clinopyroxene megacrysts can be seen as the crystallization products of host magma under the high pressure conditions.Besides,clinopyroxene megacrysts are large in size and homogeneous in composition,which requires slow crystallization under a physicochemically stable condition.Thus,clinopyroxene megacrysts are expected to have formed in an environmentally stable magma chamber.According to the calculation of temperature and pressure for clinopyroxene,the clinopyroxene megacryst formation can be concluded to that clinopyroxene(1)megacrysts in balance with host basaltic magma form as below:the volume expansion of asthenosphere magma caused by the dropping solubility of volatile components responsive to decompression damages magma conduit to form chamber in which clinopyroxene megacrysts form under a high pressure condition and are taken upon to the surface with melt by eruption;(2)megacrysts out of balance with host basaltic magma form as below:basaltic magmas derived from asthenosphere crystallize and form clinopyroxene megacrysts in a magma chamber,the megacrysts are carried up to surface by later magma.