Ophiolite in Jinsha River in Xumai-Xuedui area is widely exposed, including metamorphic peridotite, accumulation complex and plagiogranite. Its occurrence is mostly vein-like, with a width of tens of centimeters to 2 meters and an extension of tens of centimeters to several meters, in which metamorphic peridotite inclusions can be seen. The rocks are grayish white, uneven and semi-autogenous, and the main rock-forming minerals are albite (a small amount of plagioclase and feldspar).
The SHRIMP analysis results of Xumai plagiogranite Zhong 18 zircon are shown in Table 4-3 and Figure 4- 1 (Jianping, 2003), and its age of 206Pb/238U can be divided into two groups. One group is 286 ~ 3 1 1Ma, and its zircon is characterized by inconspicuous internal structure and incomplete crystal form. The other group is 2 19 ~ 258 Ma, which is characterized by self-form and developed rhythmic ring structure. On the coincidence curve, the age data points are divided into two groups: the weighted average age of 206Pb/238U is 300±5ma(n = 7) and 23910m (a n =11) respectively. This book adopts 300±5ma as the diagenetic age of Xumai plagiogranite.
Fig. 4- plagiogranite zircon U-Pb coincidence curve of snowdrift 1
(According to Jane Ping, 2003)
Zircon 4- 1 and 16- 1 also measured the age values of 807 18Ma and 634 15Ma, indicating that at least crustal zircon was captured in Xumai plagiogranite, or this plagiogranite was polluted by the crust.
2. Elephant Trunk Hill plagiogranite
Plagiogranite of Elephant Trunk Mountain is mainly distributed in the area of Elephant Trunk Mountain in Batang County. Medium-grained granite structure and fracture structure remain in the rock, and the main components are syenite (30%), plagioclase (69%, sericitization) and limonite (0.5%).
3. Niang Jiuding plagiogranite
Plagiogranite is located on Baima Snow Mountain-Yangla Highway in Deqin County, Yunnan Province, on the west side of Jinsha River in the south of Xumai plagiogranite. It is a vein with a width of about several centimeters to about 1m, which occurs in medium-fine massive gabbro, and the * * * rocks include strong serpentine pure rocks and gabbro. Plagiogranite is grayish white with medium-fine grained granite structure, and its main components are plagioclase (about 65%) and Yingshi (about 30%).
See Table 4-4 and Figure 4-2 (Jian Ping, Liu Dunyi, 2003) for the SHRIMP analysis results of plagiogranite Zhong 16 zircon, in which 15 zircon 206Pb/238U is between 373 and 309ma. In the U-Pb coincidence curve, the age data points are distributed in groups, and the weighted average age of 206Pb/238U is 285 6 Ma (n =15), which represents the crystallization age of Jiuding plagiogranite in this book. It is also found that zircon (Zircon 8- 1) with the age of 558±65438±03Ma is still polluted by the earth's crust, just like plagiogranite in Xumai.
Table 4-3 SHRIMP Analysis Results of Zircon from Xumai plagiogranite
Note: ① Error1σ; ②Pbc and Pb% represent ordinary lead and radioactive lead respectively; ③ The standard correction error is 65438 0.06%; The measured value of 204Pb is applied to ordinary lead correction.
(According to Jane Ping, 2003)
Table 4-4 Analysis Results of plagiogranite Zircon from Niangniaoding
(According to Jane Ping, 2003)
Fig. 4-2 U-Pb Consistent Curve of plagiogranite Zircon at Niangniaoding.
(According to Jane Ping, 2003)