Mineralogical Study On The Garnet Megacrysts In The Cenozoic Basalt From Maguan Yunnan Province

There are a large number of mantle xenoliths and megacrysts minerals of Cenozoic basalts in Yunnan Maguan . This paper focuses on the laser-ICP-MS analysis and the test of the garnet megacrysts of mineralogy, mineral chemistry and rare earth minerals which extracted the cause information o garnet megacrysts. At the same time, we also applied with the clinopyroxene temperature and pressure meter of the garnet megacrysts and combined with a variety of olives in this area pyroxenite rocks and mantle-type temperature andpressure results, estimating the formation of garnet megacrysts of temperature and pressureconditions. Studies showed that, in this area, garnet megacrysts has a characteristic of high aluminum magnesium in its chemical composition; by calculating the client component, we can find Pyrope,50%-69%, Almandine,16%-32%, Grossulartite,6%-12%, Andradite,1.6% -6%, Spessartite,0.441% -0.892%, Urarovite accounted for 0.045% -0.497% in the garnet megacrysts within this area; and the content of Spessartite and uvarovite is extremely low, which can be judged that the megacryst garnet is pyrope in this area,The abundance of trace elements in garnet megacrysts in this area is characterized by enrichment of the transitional metal elements, like Sc, Ti, V, Cr, Mn, Co, but the abundance of Ni is lower. The abundance of transitional metal element is mainly affected by the type and quantity of mantle peridotite source's partial melting degree and fractional crystallization of basaltic magma. The abundance of Sc mantle-derived value is much higher than the standard value of the original magma, this result is uniform to the conclusion that Sc and Cr mainly exist in garnet and clinopyroxene. However, garnet in this area also has a high V and high Ti features, that may be related with the mantle metasomatism. At the same time, this region is also enriched of garnet HFSE (eg Zr, Hf, Yb, Nb, Ti, etc.), and has a strong loss of LILE (eg Sr). The basic features of Garnet megacrysts REE's abundance in this area is, HREE (Gd-Lu) is relatively enriched, and LREE (La-Eu) losses, the distribution curve is low on the left and high on the right, basically uniform to the distribution feature of pomegranate REE stone, because the mantle rock is the main storage of garnet HREE. Garnet samples in this area has a ring part, by studying the ring structure characteristic of major elements, the preliminary analysis is that the causes of garnet megacrysts ring structure may be related with the metasomatism.By using of garnet-clinopyroxene Fe-Mg exchange meter and clinopyroxene temperature and pressure meter, referring to the temperature and pressure calculation results of the garnet's paragenetic minerals clinopyroxene mantle in this area, the temperature and pressure condition of garnet megacrysts formation had been estimated, which are 1000℃-1400℃and 1.5-2.5Gpa. According to the formula D = 4.2 +3.03 P, the formation depth of garnet megacrysts is estimated as 49.45 Km-79.75 Km. The preliminary conclusion is that the garnet megacrysts are formed by the original alkaline basaltic magma crystallization under a high mantle pressure.