Study On Garnet And Ruby From Basalts And Secondary Deposits In Thailand

Xenoliths and megacrysts are rich in the Cenozoic basalts from the ChanthaburiTrat area,southeastern Thailand.The basalts in this area are closely related to the gem corundum deposits,garnet and pyroxene megacrysts are also common around the corundum.Xenoliths and megacrysts can provide "direct samples" for studying deep effects.The research is concentrated in corundum-related deposits in southeastern Thailand,and it lacks of some study about megacrysts.A systematic study of the basalt,the xenoliths and megacrysts carried in it,and the gemstones from secondary deposits can help to explore the nearby magmatic activities and evolution.The three zones related corundum are roughly correlated with three north-south trending alkali basalt outcrops in the Chanthaburi-Trat.The basalts in the ChanthaburiTrat studied in this paper contain a large number of xenoliths,the latter also found corundum and megacrysts such as pyrope.The basalt in Bang Kacha area of Chanthaburi is gray and black.The phenocryst is dominated by olivine and the groundmass is tholeiitic texture.It is composed of plagioclase,olivine and a small amount of pyroxenes.A large number of xenocrysts can be seen under the microscope.The size is usually larger than that of phenocryst,with varying degrees of dissolution and reaction with magma,such as the ring structure of olivine,the black reaction rims of spinel,and the complex reaction rims of orthopyroxene.Basalt in the Nong Bon area of Trat contains a lot of pores.The phenocryst is dominated by olivine,which set in a glassy groundmass containing numerous tiny pyroxene microlites and magnetite granules.Xenoliths include mafic granulite xenoliths bearing-corundum and olivine xenocrysts of mantle origin.Megacrysts include a large number of black clinopyroxene and large-sized brown-red pyrope.Around megacrysts there is the reaction rims formed by the reaction with the host basalt magma: a yellowish "shell" observed around the megacrysts.The rims of clinopyroxene is sieve texture and garnet present two different reaction structures.The garnet megacrysts in the basalts from Nong Bon are similar in chemical composition to the garnet from secondary deposits,microprobe analyses of two indicate Fe-rich pyropic compositions.However,the garnets from Bo Rai are different.The gemological and spectral characteristics of the two are similar,which is consistent with the characteristics of pyrope.The ruby from secondary deposits is mostly dark purplebrown,and contains a large number of inclusions.The composition of the corresponding minerals in the ruby and the inclusions is similar to that of the xenolith.There is no direct contact,and further research is needed.