Multi-chronometer dating of the Souter Head complex: rapid exhumation terminates the Grampian Event of the Caledonian Orogeny

ABSTRACTThe Souter Head sub-volcanic complex (Aberdeenshire, Scotland) intruded the high-grade metamorphic core of the Grampian Orogen at 469.1 ± 0.6 Ma (uranium-238–lead-206 (238U–206Pb) zircon). It follows closely peak metamorphism and deformation in the Grampian Terrane and tightly constrains the end of the Grampian Event of the Caledonian Orogeny. Temporally coincident U–Pb and argon/argon (40Ar/39Ar) data show the complex cooled quickly with temperatures decreasing from ca.800 °C to less than 200 °C within 1 Ma. Younger rhenium–osmium (Re–Os) ages are due to post-emplacement alteration of molybdenite to powellite. The U–Pb and Ar/Ar data combined with existing geochronological data show that D2/D3 deformation, peak metamorphism (Barrovian and Buchan style) and basic magmatism in NE Scotland were synchronous at ca.470 Ma and are associated with rapid uplift (5–10 km Ma−1) of the orogen, which, by ca.469 Ma, had removed the cover to the metamorphic pile. Rapid uplift resulted in decompressional melting and the generation of mafic and felsic magmatism. Shallow slab break-off (50–100 km) is invoked to explain the synchroneity of these events. This interpretation implies that peak metamorphism and D2/D3 ductile deformation were associated with extension. Similarities in the nature and timing of orogenic events in Connemara, western Ireland, with NE Scotland suggest that shallow slab break-off occurred in both localities.

The Turner Mountain syenite, Maine, USA: geology, geochemistry, geochronology, petrogenesis, and post-orogenic exhumation

The Turner Mountain syenite is one of the few plutons located entirely within the Norumbega fault system in the northern Appalachian orogen. It is composed of texturally and mineralogically homogeneous biotite-amphibole syenite and is in faulted contact with mylonitic leucogranite and an unmetamorphosed redbed unit. It is intermediate in SiO2 content (58.7–65.1 wt%) and ultrapotassic (6.4–7.9 wt% K2O) with high K2O/Na2O ratios (2.75–4.15), yet is relatively primitive in terms of MgO (2.8–4.9 wt%), Ni (average 90.2 ppm), and Cr (average 210.1 ppm) contents. It has enriched large-ion-lithophile elements relative to high field strength elements, high contents of light rare-earth elements, and initial 87Sr/86Sr ratios (0.7038–0.7068) similar to that of OIB basalts. It differs petrologically and geochemically from the neighboring Lucerne-Deblois plutons but is similar to Lincoln syenite located 100 km to the southwest, also within the Norumbega system. Zircon U-Pb dating using LA-ICP-MS yields a weighted mean age of 410.5 ± 2.4 Ma, slightly younger than the Lincoln syenite (418 ± 1 Ma). Based on their distinctive geochemical signatures, both were probably products of Late Silurian-Early Devonian ultrapotassic magmatism related to Acadian subduction, generated by partial melting of a mantle wedge metasomatized by potassium-rich fluids during west-directed subduction. This unique magmatism could be attributed to decompressional melting during Late Silurian-Early Devonian slab break-off or delamination. Based on Sr-Nd isotopic compositions, the Turner Mountain syenite magma probably had more crustal interaction than that which formed the Lincoln syenite. The syenite was later exhumed tectonically during brittle reactivation of the Norumbega fault. The reactivation involved regional-scale, high-angle, southeast-over-northwest reverse faulting in a transpressional environment and occurred during the Late Devonian and through Mississippian to Permian.

Longevity of the Permian Emeishan mantle plume (SW China): 1 Ma, 8 Ma or 18 Ma?

After the formation of the ~ 260 Ma Emeishan large igneous province, there were two volumetrically minor magmatic pulses at ~ 252 Ma and ~ 242 Ma, respectively. Alkaline mafic dykes intruding both 260 Ma and 252 Ma felsic plutons in the Panxi region, southwestern China, have compositions similar to the Emeishan flood basalts. One dyke is dated using the SHRIMP zircon U–Pb technique at 242 ± 2 Ma, ~ 18 Ma younger than the start of Emeishan magmatism. The dykes have enriched light rare earth element patterns (La/YbN = 4.4–18.8) and trace element patterns similar to the Emeishan flood basalts and average ocean-island basalts. Some trace element ratios of the dykes (Zr/Nb = 3.8–8.2, La/Nb = 0.4–1.7, Ba/La = 7.5–25.6) are somewhat similar to EM1 source material, however, there are differences. Their εNd values (εNd = +2.6 and +2.7) andISr (ISr = 0.704542 and 0.704554) ratios are indicative of a mantle source. Thus Emeishan magmatism may have lasted for almost 20 Ma after the initial eruption. However, geological evidence precludes the possibility that the post-260 Ma magmatic events were directly related to Emeishan magmatism, which began at and ended shortly after 260 Ma. The 252 Ma plutons and 242 Ma dykes represent volumetrically minor melting of the fossil Emeishan plume-head beneath the Yangtze crust. The 252 Ma magmatic event was likely caused by post-flood basalt extension of the Yangtze crust, whereas the 242 Ma event was caused by decompressional melting associated with the collision between the South China and North China blocks during the Middle Triassic.

GEOCHEMICAL CHARACTERISTICS OF PIGI AND KARATHODORO MIGMATIZED METASEDIMENTS, GUEVGUELI COMPLEX, MACEDONIA, GREECE

Two continental slivers, those of Pigi and Karathodoro (parts of the Stip-Axios unit) are intercalated among members of the Guevgueli ophiolitic complex. These two slivers are closely related with the Late Jurassic basic and acid (expressed mainly by Fanos granite) magmatism ofthat area. A subduction-related volcanic suite intrudes the two continental slivers. Migmatized metasediments constitute the main lithological type of Pigi and Karathodoro. The geochemistry shows that minerals participating in the main decompressional-melting reactions, such as biotite, sillimanite, cordierite, plagioclase and orthoclase, control the differentation of the major elements between mesosomes - diatexites, leucosomes and restites. The refractory behavior of accessory minerals, such as monazite, allanite, zircon, hercynite and ilmenite, control the differentiation of the trace elements. Harker and rare earth element diagrams establish a complementary relationship between restites, leucosomes and mesosomes. In the peraluminous system Ab-Qz-Or leucosomes are projected close to the thermal minimum between 760-800 °C. A main exception is the Platania granite projected within the quartz stability field, probably due to the participation of restitic quartz in its mineralogy. The geochemical results are in good agreement with the pétrographie observations and show that the partial melting was responsible for the formation of Pigi and Karathodoro migmatites