| Citation: |
TIAN Zhibo, GOU Longlong, XU Xiaofei, LIU Xuefeng, MAO Zhenyu. 2025. Metamorphic P–T Conditions and In–situ Rb–Sr Geochronology of the Kuanping Group in the Laoyu Area of the Qinling Orogenic Belt. Northwestern Geology, 58(1): 1-26. doi: 10.12401/j.nwg.2023046
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Metamorphic P–T Conditions and In–situ Rb–Sr Geochronology of the Kuanping Group in the Laoyu Area of the Qinling Orogenic Belt
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Abstract
The laoyu area of the Qinling orogenic belt has a typical section of the Kuanping group, which is important for studying the metamorphism, deformation, and tectonothermal history of the Kuanping group. However, the metamorphic P–T conditions and chronology of the Kuanping group in this region are still lacking, which hinders our understanding of the relationship between its regional metamorphism and later deformation events, as well as their geological significances. In this study, detailed petrographic studies were carried out on two–mica quartz schist, garnet–bearing two–mica quartz schist, greenschist, and marble in the north–south section of the Kuanping group in this area. Based on this, the geological significances of multiple tectonothermal events that the Kuanping group in the Laoyu region underwent were examined with a focus on two-mica quartz schist and garnet–bearing two–mica quartz schist using Ti–in–biotite thermometry, phengite geobarometry, phase equilibrium modelling, and in situ LA–ICP–MS biotite and muscovite Rb–Sr dating. According to field and petrographic observations, two–mica quartz schist and greenschist were both significantly deformed, and phlogopite marble suffered strong mylonitization. The Ti–in–biotite thermometer and phengite geobarometer yielded the metamorphic PT conditions of 300～500 ℃ and 2.0～8.0 kbar (average values are 440 ℃ and 4.0 kbar) for the two–mica quartz schist samples KP-3 and KP-4. The Ti–in–biotite thermometry constrained the metamorphic temperature of the garnet–bearing two–mica quartz schist sample KP2202 to be 652～683 ℃. According to the PT pseudosection modeling, the metamorphic PT conditions of the two–mica quartz schists and and the garnet–bearing two–mica quartz schists are 400～480 ℃ and 2.0～10 kbar, and 645～680 ℃ and 8.0～9.0 kbar, respectively. On the basis of the results from the geothermobarometry and phase equilibrium modelling, the two–mica quartz schist is the consequence of greenschist–facies metamorphism, whereas the garnet–bearing two–mica quartz schist formed by low–amphibolite facies metamorphism. In–situ LA–ICP–MS biotite and muscovite Rb–Sr dating shows that the two–mica quartz schist records two isochron ages of ～290 Ma and ～155 Ma, while the garnet–bearing two–mica quartz schist records an isochron age of ～110 Ma. Consequently, the two–mica quartz schists in the Kuanping group of the Laoyu region record three isochron ages, which are ～290 Ma, ～155 Ma, and ～110 Ma. Combined with the results of previous studies, all three isochron ages represent the timings of late tectonothermal events, where the isochron age of ～290 Ma corresponds to the northward subduction of the paleo–Tethys Oceanic crust, while the isochron ages of ～155 Ma and ～110 Ma may be related to the intense deformation and thermal resetting caused by granitic magmatism in the North Qinling tectonic belt during the Mesozoic.
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References
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TIAN Zhibo, GOU Longlong, XU Xiaofei, LIU Xuefeng, MAO Zhenyu. 2025. Metamorphic P–T Conditions and In–situ Rb–Sr Geochronology of the Kuanping Group in the Laoyu Area of the Qinling Orogenic Belt. Northwestern Geology, 58(1): 1-26. doi: 10.12401/j.nwg.2023046
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Figure 1.
Simplified tectonic division of the Qinling orogenic belt
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Figure 2.
Simplified geological map of Laoyu area, showing the with sample location
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Figure 3.
Photographs of outcrop
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Figure 4.
Photomicrographs of the two-mica quartz schist and the garnet-berting two-mica quartz schist
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Figure 5.
(a) Biotite Mg–Fe2+ diagram, (b) plagioclase XOr–XAb–XAn diagram in the garnet-bearing two-mica quartz schist sample KP2202
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Figure 6.
Compositional profiles of garnet porphyroblast from the garnet-bearing two-mica quartz schist sample KP2202
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Figure 7.
Photo-micrographs of green schist and mylonitized phlogopite-bearing marble
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Figure 8.
The P-T conditions calculated by (a) Ti-in-biotite geothermometer and (b) phengite geobarometer
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Figure 9.
(a) P–T pseudosection, (b) P–T pseudosection with isopleths of XTi(bt) and XFe(bt) for the two-mica quartz schist sample KP-3
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Figure 10.
(a) P–T pseudosection, (b) P–T pseudosection with isopleths of XTi(bt) and XFe(bt) for the two-mica quartz schist sample KP-4
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Figure 11.
(a) P–T pseudosection, (b) P–T pseudosection with isopleths of XPy and XGrs for the garnet-bearing two-mica quartz schist sample KP2202
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Figure 12.
Spot locations for in-situ LA-ICP-MS biotite and muscovite Rb–Sr analysis
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Figure 13.
In situ LA–ICP–MS biotite and muscovite Rb–Sr isochron diagrams
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Figure 14.
Summary of metamorphic P–T conditions and paths for the Kuanping Group