Nature of the source regions for post-collisional, potassic magmatism in southern and northern Tibet from geochemical variations and inverse trace element modelling

Neogene potassic lavas in northern and southern Tibet have different isotopic (epsilonNd((i)) north, -5.5 to -10.3; south -8.8 to -18.1) and major element signatures suggesting derivation from separate sub-continental lithospheric mantle (SCLM) sources. Inverse trace-element modelling shows that the southern Tibet magmas were derived by 1-2% partial melting of a phlogopite and amphibole peridotite, and that the northern samples were derived by 3-4% partial melting of a phlogopite peridotite. In both cases, melting is inferred to take place in the spinel stability field. Both sources show large ion lithophile element (LILE) enrichment relative to the high field strength elements (HFSE), and heavy rare earth element (HREE) depletion relative to primitive mantle. LILE/HFSE enrichment suggests subduction-related metasomatism; HREE depletion is indicative of prior melt extraction. Extension postdates the earliest magmatism in southern and north-central Tibet by 7 Myr and 5 Myr, respectively, which, in combination with the shallow depths of melting inferred for the Tibetan samples, supports geodynamic models invoking thinning of the SCLM. The northern Tibetan magmatism and extension can be explained by convective removal of the lower SCLM; the older ages and arcuate distribution of the southern magmas are most consistent with the SCLM erosion following slab break-off

Tectonic implications of Palaeoproterozoic anatexis and Late Miocene metamorphism in the Lesser Himalayan Sequence, Sutlej Valley, NW India

Unravelling the kinematic evolution of orogenic belts requires that the defining tectonostratigraphic units, and structural elements that bound them, are properly identified and characterized. In the Sutlej Valley (western Himalaya), the Munsiari and Vaikrita thrusts have both been correlated with the Main Central Thrust. The sequence of amphibolite-grade rocks (the Jutogh Group) bounded by these faults has been variously assigned to the Lesser Himalayan Sequence (based on provenance ages) and to the Greater Himalayan Sequence (from its metamorphic grade). Trace-element and geochronological data from leucogranites in the Jutogh Group (1) indicate crustal melting at c. 1810 Ma, before the deposition of the Greater Himalayan Sequence, thus correlating the Jutogh Group with the Lesser Himalayan Sequence, and (2) record Proterozoic metamorphism overprinted at 10.5 ± 1.1 Ma (established from U–Pb analysis of uraninite) during the Himalayan orogeny. Pressure–temperature–time data indicate that the Jutogh Group and Greater Himalayan Sequence represent distinct tectonic units of the metamorphic core that were decoupled during their extrusion. This precludes extrusion along a single, widening channel, and requires a southward shift of the locus of movement during the Late Miocene, coincident with present-day precipitation patterns

Tectonic implications of Palaeoproterozoic anatexis and Late Miocene metamorphism in the Lesser Himalayan Sequence, Sutlej Valley, NW India

<p>Unravelling the kinematic evolution of orogenic belts requires that the defining tectonostratigraphic units, and structural elements that bound them, are properly identified and characterized. In the Sutlej Valley (western Himalaya), the Munsiari and Vaikrita thrusts have both been correlated with the Main Central Thrust. The sequence of amphibolite-grade rocks (the Jutogh Group) bounded by these faults has been variously assigned to the Lesser Himalayan Sequence (based on provenance ages) and to the Greater Himalayan Sequence (from its metamorphic grade). Trace-element and geochronological data from leucogranites in the Jutogh Group (1) indicate crustal melting at <em>c</em>. 1810 Ma, before the deposition of the Greater Himalayan Sequence, thus correlating the Jutogh Group with the Lesser Himalayan Sequence, and (2) record Proterozoic metamorphism overprinted at 10.5 ± 1.1 Ma (established from U–Pb analysis of uraninite) during the Himalayan orogeny. Pressure–temperature–time data indicate that the Jutogh Group and Greater Himalayan Sequence represent distinct tectonic units of the metamorphic core that were decoupled during their extrusion. This precludes extrusion along a single, widening channel, and requires a southward shift of the locus of movement during the Late Miocene, coincident with present-day precipitation patterns. </p