Determination of Early Paleozoic metamorphic event in the Longshan Complex and its geological implication

The tectonic affinity of the Longshan Complex has been controversial for a long time. In this paper, detailed petrographical observations, mineral chemistry, pressure-temperature (P-T) calculations and monazite and titanite chronological dating were conducted on the garnet-biotite plagioclase gneisses and garnet-bearing amphibolites from the Longshan Complex in the Qinling-Qilian connection zone. Based on petrographical observation, the peak metamorphic mineral assemblage (garnet + biotite + plagioclase + quartz) and the peak metamorphic mineral assemblage (garnet + clinopyroxene + hornblende + plagioclase + quartz) were recognized in the garnet-biotite plagioclase gneisses and the garnet-bearing amphibolites, respectively. By the traditional geothermobarometer P-T calculations, the peak metamorphic P-T conditions of samples 21LS40 and 21LS42-1 were constrained at 700 degrees C, 0.72GPa and 710 degrees C, 0.74GPa, respectively. Monazite laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb data from the Longshan metapelites indicate that the weighted average Pb-206/U-238 ages of measured samples are 407 similar to 435Ma. Titanites from the garnet-bearing amphibolites yield low intercept U-Pb ages of 410 +/- 7Ma, 409 +/- 5Ma and 426 +/- 10Ma, and the correspondent Zr-in-titanite temperatures were 750 degrees C 751 degrees C and 748 degrees C at 0.7GPa. By comparing with the metamorphic ages and P-T conditions of the Qinling Complex, we preliminarily advocate that the Longshan Complex is similar to the last stage retrograde metamorphism of the HP-UHP rocks from East Qinling Complex and the granulite facies metamorphism of the West Qinling Complex from the Tianshui area, thus, the Longshan Complex may belong to the western extension of the North Qinling orogenic belt.