Geochemistry And Petrogenesis Of Enclaves And Their Host Granite In Fengqing County,yunnan Province

A large number of dark enclaves with different morphology and compositions have been found in Fengqing granite, in northwestern of Lincang granite batholith, and which have become the natural probe for studying on deep geological processes in this region.It is found that the rock mass is mainly composed of biotite monzonitic granite.The enclaves are mostly in the form of teardrop or ovoid shape and most of the contact boundaries between the host granite and enclaves are clear comparatively, the rest of it are transitionary. In comparison with the host rocks, the colour of enclaves is more dark, and the mineral granularity of enclaves is even finer. And the majority of enclaves have the typical granitic texture, and develop a kind of structure of plagioclase which has several kinds of special textures such as zonate texture, poikilitic texture and irregular proliferous margin texture. The mineral chemical characteristics of the host rocks and encalves show that,the magma sources of this region are mantle or crust-mantle mixed. Feldspar include plagioclase and potassium feldspar. Among them, most of plagioclase are andesine(An=27-49.86).The content of Si O2 in the host rocks ranges from 67.02% to 68.02%, and the average is 66.99%.In addition, the content of Al2O3 is high, the average is 15.37%,which is mainly concentrate in 15.80% to 15.01%. The content of Ti O2 is relatively low, which is between 0.52% and 0.813%, the average of it is 0.64%. The K2O/Na2 O ratio ranges from 1.37% to 1.91%. The host rocks belong to high-K calc-alkaline peraluminous-strongly peraluminous rocks(A / KNC=1.47-2.28, 1.81 on average, σ=0.65-2.77, 1.74 on average); the enclaves belong to calc-alkaline peraluminous rocks that its’ A/KNC ratio and rittman index(σ) are slightly lower than the host rocks’(A/KNC=1.55-1.73, 1.63% on average; σ = 0.65-2.77, 1.61 on average). The TFe O/(TFe O+Mg) ratio of the host rocks and enclaves are very similar, some other ratios of the major oxides,such as Si O2/Ca O-K2O/Ca O、(Fe2O3+ Fe O)/ Si O2-K2O/Ca O,also show a covariant relationship obviously. In the K2O-Na2 O discrimination diagram,their projection points are both in the zone of S-type granites. Some covariance plots of trace element also display a significant linear correlation, which indicate heterogeneous magma mixing and melting evolution relationship, to a certain extent,between the host granites and enclaves.The chondrite-normalized REE patterns show obvious right slanting, relatively enriched LREE and depleted HREE. In addition, which has the obvious characteristic: negative Eu anomaly with δEu of 0.42-0.56, 0.48 on average.The totel content of REE is between 198.53×10-6- 318.33×10-6, with(La/Yb)N ratios of 12.50-18.87,and the average is 15.31. The host rocks and enclaves have similar patterns of trace element and REE distribution curve, which reflect that they have some definite relationships in geochemical composition. Therefore, it can be speculated that both of them have experienced a similar process about magmatic evolution. Comparatively, Rb/Sr ratio of enclaves is more higher(0.73-1.55), the average of it is 1.01,which suggested that the enclaves could form in this process:the intermediate or basic magma mixed with the acid or acidic magma, as a result of the different composition and temperature and other factors obviously, causing magma rapid condensation and crystallization.In addition, in the plots of tectonic discrimination such as Y-Nb and Yb+Ta-Rb indicate that the rocks in this region have multiple characteristics of continental intraplate granite, arc volcanic granite and syn-collision granite, which is suggested that the formation environment belongs to transitional-type continental margin orogenic belts.