Central Iberian Variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous, high-K calc-alkaline, ultrabasic to intermediate rock series. Despite their overall calc-alkaline affinity, the mafic–ultramafic members contain high-Ti amphibole oikocrysts rimmed by lower-Ti amphibole ± cummingtonite and high-Ti amphibole replacing early phlogopite. To understand the factors controlling the saturation of high-Ti amphibole in the parental magmas, clinopyroxene-melt, phlogopite-melt and amphibole-melt relationships are reviewed. This analysis reveals that for melts with intermediate compositions, the affinity of TiO2 for amphibole rises in alkalic magmas. Accordingly, mildly alkalic trachytoid to subalkaline medium- to high-K andesite and dacite compositions are estimated for interstitial high-Ti amphibole-saturated melts. Amphibole Ce/Pb ratios reveal a mantle–crust hybrid nature for interstitial melts with subalkaline trachytoid compositions. The hydrous character of the Variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of Na2O with respect to silica during amphibole crystallization.

High-Ti amphibole as a petrogenetic indicator of magma chemistry: evidence for mildly alkalic-hybrid melts during evolution of Variscan basic–ultrabasic magmatism of Central Iberia

Evidence from Antarctic mantle peridotite xenoliths for changes in mineralogy, geochemistry and geothermal gradients beneath a developing rift

[1] The denudational history along a transect across the western margin of the Lambert Graben, northern Prince Charles Mountains, East Antarctica, is quantified on the basis of thermal history modeling of apatite fission track data from four vertical profiles. Early Cretaceous paleogeothermal gradients estimated from the fission track data increase from ∼19°C km−1 ∼50 km west of the rift margin to ∼29°C km−1 within the immediate vicinity of the western master fault. Two discrete phases of enhanced denudation during the Phanerozoic are inferred for the western rift margin. The first occurred during the early Paleozoic, with minimum denudation of between 1.6 and 5.0 km, which is broadly coeval with initial rifting and the major period of Permo-Triassic sedimentation within the Lambert Graben. The formation of the graben was related to the formation of the ancestral Gamburtsev Mountains due to Variscan compression and substantial crustal thickening of the East Antarctic Craton. The second phase of denudation commenced in the Early Cretaceous, with estimated amounts ranging between 1.0 and 4.5 km, generally increasing toward the rift margin, with the maximum total denudation occurring ∼20 km west of the western master fault of the graben. The Early Cretaceous phase of denudation was probably related to tectonic reactivation of the earlier Permo-Triassic rift during the initial separation of the Indian and Antarctic sector of Gondwana.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002TC001477

Denudational and thermal history along a transect across the Lambert Graben, northern Prince Charles Mountains, Antarctica, derived from apatite fission track thermochronology

Brittle faulting in the Prince Charles Mountains, East Antarctica: Cretaceous transtensional tectonics related to the break-up of Gondwana

The geological composition of the hidden Wilhelm II Land in East Antarctica: SHRIMP zircon, Nd isotopic and geochemical studies with implications for Proterozoic supercontinent reconstructions

A compositionally varied swarm of mafic dykes in the Jetty Peninsula area was emplaced about 320 Ma ago (K-Ar age). There are three major groups: Group 1 dykes range from transitional-alkaline dolerites to camptonites, Group 2 are trachydolerites, and Group 3 are diorite to quartz diorite porphyries. Group 1 dykes have very similar ratios of most incompatible elements and were derived from the same (or a very similar) enriched lithospheric mantle source region (∈ Nd −0.18 to −3.05) with high Nb and Ta (i.e., OIB, ocean island basalt, characteristics). However, the presence of several distinct subgroups with different incompatible element abundances implies significantly different degrees of melting. Group 2 trachy dolerites are much more fractionated ( mg 22–36), but were apparently derived from a similar, although somewhat more enriched (∈ Nd −2.26 to −4.63) source. Group 3 diorites are compositionally quite distinct and may have been derived by intracrustal melting. Enrichment of the mantle source(s) of Groups 1 and 2 dykes apparently occurred about the same time as high-grade metamorphism in the area, and may have been coeval with crust formation in nearby parts of Gondwana.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0954102093000392

Eugene V. Mikhalsky

Papers

Association of dolerite and lamprophyre dykes, Jetty Peninsula (Prince Charles Mountains, East Antarctica)