2:30 PM - 2:45 PM
[SMP28-16] Deformation of migmatites and leucogranites from the Pangong Metamorphic Complex of the Indian Karakoram Himalaya
Keywords:Crystallographic preferred orientation, Deformation mechanisms, Partial melting, Seismic anisotropy
Partial melting and melt-rock interactions are two mutually linked processes that fundamentally affect the deformation of the continental crust. Migmatites and leucogranites, the products of partial melting, are ideally suited to investigate the effects of melt presence and migration on the rheological and geochemical characteristics of crustal rocks. When partial melting and deformation are synchronous, the latter controls the arrangement of the melt networks, which inhibit intergranular stress transfer, abate ductile deformation of the constituent mineral grains, and dictate the overall deformation of the rocks. Water-fluxed melting is common within shear zones because the reduced pressure inside an active shear zone produces the gradient that facilitates fluid invasion. The Karakoram Shear Zone (KSZ) from the northwestern Indian Himalaya is one such example that acted as a pathway for aqueous fluids, which in turn triggered partial melting of the calc-alkaline magmatic and biotite-rich metasedimentary rocks and produced the widespread migmatites and leucogranites in the region. Towards the east, the 10km wide Pangong Metamorphic Complex (PMC) is exposed within the KSZ and is bounded by the steep Tangtse and Pangong Shear Zones in the SW and NE, respectively.
We investigated the deformation characteristics of three amphibolites (LD29, 36B, and 43B) and four quartzo-feldspathic leucogranites (LD30, 33A, 40, 43A) from the PMC. Mineralogically, most amphibole grains in the amphibolites are magnesio-hornblende. Some amphiboles in LD36B, collected from the Pangong Shear Zone, are actinolitic. The TAl content of the amphiboles from this sample is more diverse (0.32-1.34 apfu) compared to those (TAl = 1.06-1.48) in LD29, collected from the core of the PMC. The Na occupancies at the B-sites in the amphiboles of LD29 are >0.2 apfu, whereas for those in LD36B are <0.2 apfu. The amphibole grains in the amphibolites exhibit strong shape preferred orientations and define the foliation. The amphiboles also exhibit strong crystallographic preferred orientations (CPOs), with the <001> and <100> axes being mainly distributed parallel to the XZ and YZ planes, respectively. The <010> CPOs are relatively weaker. In this respect, the observed amphibole CPOs differ from the four types demonstrated using experimental deformations. Further studies will be carried out to understand the origin of these CPOs. In any case, the CPOs and low-angle misorientation axes distributions suggest that the dominant slip systems are <001>(010) in LD29 and LD43B, whereas in LD36B, (001) appears to be the main slip plane, which is unreported. The quartz CPOs are weaker in LD30, collected from the core of the PMC. The CPOs become stronger away from the core, with those in LD43A, near the Pangong Shear Zone, being the strongest. Prism-a is the dominant slip system observed for the quartz grains in all the samples, suggesting deformation temperatures of 400-600°C. The plagioclase grains exhibit moderate to strong CPOs overall, with those in the amphibolite samples showing stronger CPOs than the ones present in the leucogranites. <010>(001) slip is prominent in the biotite grains of LD30 and LD40. The strong CPOs, subgrain boundaries, and crystallographic control on the distributions of the low-angle misorientation axes indicate that the amphibole, plagioclase, and quartz grains have deformed via dislocation creep mechanisms in the absence of pervasive grain boundary melt. The bulk rock P-wave and fast S-wave anisotropies of the amphibolites range from 11.2-12.2 and 7.6-13.4 km s-1, respectively. The P-wave velocities are the slowest and fastest parallel to the <100> and <001> axes of the constituent amphibole grains. The bulk rock fast S-wave polarization direction is sub-normal to the foliation in LD36B but moderately oblique in LD29 and LD43B. It is also nearly perpendicular to the lineation in all three amphibolites.
We investigated the deformation characteristics of three amphibolites (LD29, 36B, and 43B) and four quartzo-feldspathic leucogranites (LD30, 33A, 40, 43A) from the PMC. Mineralogically, most amphibole grains in the amphibolites are magnesio-hornblende. Some amphiboles in LD36B, collected from the Pangong Shear Zone, are actinolitic. The TAl content of the amphiboles from this sample is more diverse (0.32-1.34 apfu) compared to those (TAl = 1.06-1.48) in LD29, collected from the core of the PMC. The Na occupancies at the B-sites in the amphiboles of LD29 are >0.2 apfu, whereas for those in LD36B are <0.2 apfu. The amphibole grains in the amphibolites exhibit strong shape preferred orientations and define the foliation. The amphiboles also exhibit strong crystallographic preferred orientations (CPOs), with the <001> and <100> axes being mainly distributed parallel to the XZ and YZ planes, respectively. The <010> CPOs are relatively weaker. In this respect, the observed amphibole CPOs differ from the four types demonstrated using experimental deformations. Further studies will be carried out to understand the origin of these CPOs. In any case, the CPOs and low-angle misorientation axes distributions suggest that the dominant slip systems are <001>(010) in LD29 and LD43B, whereas in LD36B, (001) appears to be the main slip plane, which is unreported. The quartz CPOs are weaker in LD30, collected from the core of the PMC. The CPOs become stronger away from the core, with those in LD43A, near the Pangong Shear Zone, being the strongest. Prism-a is the dominant slip system observed for the quartz grains in all the samples, suggesting deformation temperatures of 400-600°C. The plagioclase grains exhibit moderate to strong CPOs overall, with those in the amphibolite samples showing stronger CPOs than the ones present in the leucogranites. <010>(001) slip is prominent in the biotite grains of LD30 and LD40. The strong CPOs, subgrain boundaries, and crystallographic control on the distributions of the low-angle misorientation axes indicate that the amphibole, plagioclase, and quartz grains have deformed via dislocation creep mechanisms in the absence of pervasive grain boundary melt. The bulk rock P-wave and fast S-wave anisotropies of the amphibolites range from 11.2-12.2 and 7.6-13.4 km s-1, respectively. The P-wave velocities are the slowest and fastest parallel to the <100> and <001> axes of the constituent amphibole grains. The bulk rock fast S-wave polarization direction is sub-normal to the foliation in LD36B but moderately oblique in LD29 and LD43B. It is also nearly perpendicular to the lineation in all three amphibolites.