Zircon SHRIMP U-Pb geochronology, geochemistry and petrogenesis of the upper Eocene Shuangmaidi peraluminous granite in Baoshan Block, Western Yunnan Terrain, southwestern China

Abstract

The Baoshan Block is tectonically located in the middle segment of the Sibumasu plate. Granitic magmatism within the Baoshan Block has been considered weakly active due mainly to very limited exposures during the Himalaya orogenic episode. The geochronological study on the buried Shuangmaidi granite has confirmed the existence of the Cenozoic granitoids in the Baoshan Block. The present study indicates that: (1) It is medium- to coarse-grained two mica phyric granite, characterized by high SiO2 (73.55%–77.16%) and low CaO (0.34%–1.38%) contents, with a total alkalis (K2O+Na2O) of 5.22%–8.03%, K2O/Na2O ratios of 0.24–1.79, and total rare earth elements (ΣREE) of the granite between 85 and 125 ppb. All samples are enriched in light REE and exhibit medium negative Eu anomalies; and they show pronounced negative anomalies in Ba, Sr, Ti, and Nb but significant positive anomalies in K, Rb, U, Th, and Pb on mantle-normalized trace element patterns, indicating typically peraluminous to strongly peraluminous S-type granite. (2) The zircon SHRIMP U-Pb ages of the granite are 36.27±0.48 Ma for the samples from ZK7-1 and 35.78±0.49 Ma for those from ZK0-1, respectively. The similar zircon ages from these two drill cores may suggest that the granite samples come from the same buried pluton. (3) 206Pb/204Pb values of the granite vary from 20.115 to 25.359, 207Pb/204Pb from 15.776 to 16.160, and 208Pb/204Pb from 39.236 to 41.285, showing the characteristics of radioactive lead anomaly of the upper crust. The (87Sr/86Sr)i values calculated on the average age of the two-mica orthoclase granite (36 Ma) range from 0.72524 to 0.77503 and ɛNd(t) values vary from −10.9 to −11.7. These data, along with the depleted-mantle Nd modal ages of 1.73–1.80 Ga, imply that the granites might have formed from partial melting of the Precambrian crystal basements. (4) On the Hf-Rb-Ta diagram, almost all the samples fall within the field of post-collision tectonic setting. The CaO/Na2O and Al2O3/TiO2 ratios suggest that the granitic magma may have formed from partial melting of clay-rich crustal materials with a possible melting temperature of about 900°C and a possible crystallization temperature of 775–795°C. (5) During the post-collision of the Himalaya orogen, with the southeastward extrusion of the Indochina continent resulting from the continuous northward indentation of the India continent into the Asia, the Gaoligong Fault, as the western boundary of the Indochina continent, moved in the dextral strike-slip on a large scale to trigger partial melting of the thickened crust, and the peraluminous granitic magma from which the Shuangmaidi two-mica orthoclase granite derived was formed.