Prograde and retrograde growth of monazite in migmatites:An example from the Nagercoil Block,southern India

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time （P-T-t） evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6-8 kbar and temperatures in excess of 900 ℃. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 ℃. SHRIMP U-Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palae- oproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P-T-t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

Reappraising the P-T evolution of the Rogaland-Vest Agder Sector,southwestern Norway

The Rogaland-Vest Agder Sector of southwestern Norway comprises high-grade metamorphic rocks intruded by voluminous plutonic bodies that include the ~1000 km^2 Rogaland Igneous Complex（RIC）.New petrographic observations and thermodynamic phase equilibria modelling of three metapelitic samples collected at various distances（30 km,10 km and ~ 10 m） from one of the main bodies of RIC anorthosite were undertaken to assess two alternative P-T-t models for the metamorphic evolution of the area.The results are consistent with a revised two-phase evolution.Regional metamorphism followed a clockwise P-T path reaching peak conditions of ~ 850-950 ℃ and ~7-8 kbar at ~1035 Ma followed by high-temperature decompression to ~5 kbar at ~950 Ma,and resulted in extensive anatexis and melt loss to produce highly residual rocks.Subsequent emplacement of the RIC at ~930 Ma caused regional-scale contact metamorphism that affected country rocks 10 km or more from their contact with the anorthosite.This thermal overprint is expressed in the sample proximal to the anorthosite by replacement of sillimanite by coarse intergrowths of cordierite plus spinel and growth of a second generation of garnet,and in the intermediate（10 km） sample by replacement of sapphirine by coarse intergrowths of cordierite,spinel and biotite.The formation of late biotite in the intermediate sample may suggest the rocks retained small quantities of melt produced by regional metamorphism and remained at temperatures above the solidus for up to 100 Ma.Our results are more consistent with an accretionary rather than a collisional model for the Sveconorwegian Orogen.

Potassium isotopes trace the formation of juvenile continental crust

Constraining the processes associated with the formation of new(juvenile)continental crust from mantle-derived(basaltic)sources is key to understanding the origin and evolution of Earth’s landmasses.Here we present high-precision measurements of stable isotopes of potassium(K)from Earth’s most voluminous plagiogranites,exposed near El-Shadli in the Eastern Desert of Egypt.These plagiogranites exhibit a wide range of d41K values(–0.31‰±0.06‰to 0.36‰±0.05‰;2 SE,standard error)that are significantly higher(isotopically heavier)than mantle values(–0.42‰±0.08‰).Isotopic(87Sr/86Sr and^(143)Nd/^(144)Nd)and trace element data indicate that the large variation in d41K was inherited from the basaltic source rocks of the El-Shadli plagiogranites,consistent with an origin through partial melting of hydrothermally-altered mid-to-lower oceanic crust.These data demonstrate that K isotopes have the potential to better constrain the source of granitoid rocks and thus the secular evolution of the continental crust.

Newly-discovered ultra-high temperature granulites from the East Hebei terrane,North China Craton

Archean crustal terranes are commonly featured with the dome–and–keel structures,in which supracrustal rocks occur as belts(greenstones)between,and rafts within,dome-like bodies of tonalite–trondhjemite–granodiorite(TTG).Supracrustal rafts within TTG gneisses generally record metamorphic grades ranging from amphibolite-to ultra-high temperature(UHT)granulitefacies of low-to moderate-pressure conditions[1].Notably,granulite-facies rafts commonly show counterclockwise P±T paths involving a significant increase in pressure at high temperatures[2,3].However,the tectonothermal drivers behind such metamorphism are still debated.