Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps.

We report SHRIMP zircon U-Pb ages for post-Usagaran granitic- to granodioritic intrusives and a rhyolitic agglomerate from the Palaeoproterozoic terrain of southwestern Tanzania. This terrain consists of strongly deformed and metamorphosed rocks ascribed to the ca. 2 Ga Usagaran mobile belt, voluminous post-Usagaran granitoids, and minor supracrustal successions. The southeastern part of this terrain is characterized by the occurrence of little deformed and virtually unmetamorphosed rhyolithic, dacitic and andesitic volcanic rocks which overlie the older Usagaran basement. These rocks extruded between 1820 and 1921 Ma, as documented by SHRIMP zircon ages, which are in good agreement with previous ages reported from this region, from the Ubendian belt farther west, and from Palaeoproterozoic crustal fragments in the Neoproterozoic Pan-African Mozambique belt to the east. A 2519Ò9 Ma Archaean zircon xenocryst was found in one volcanic rock sample, also in good agreement with Archaean ages of the Tanzania craton to the northwest and Archaean rocks in the central part of the Mozambique belt. Our age data suggest that the post-Usagaran assemblage, including crustal domains farther east in the MB, was generated during a relatively short period of ~100 Ma in the late Palaeoproterozoic, probably during a crustal accretion event connected with widespread magmatic activity and generally ascribed in Africa to the Eburnian orogeny.

Flood basalts of the Deccan Volcanic Province erupted between about 67.5 to 60.5 Ma ago and reached a thickness of up to 3500 m. The main part consists of compound and simple lava flows with a tholeiitic composition erupted within 500,000 years at about 65 Ma. Within the compound lava flows, vesicles and cavities are frequent. They are filled by secondary minerals partly of well development and large size. This study presents data on the secondary mineralization including detailed field descriptions, optical, cathodoluminescence and SEM microscopy, X-ray diffractometry, fluid inclusions, C and O isotope analyses, and Rb-Sr and K-Ar geochronology. The investigations indicate a multistage precipitation sequence with three main stages. During stage I clay minerals and subsurface filamentous fabrics (SFFs), of probably biogenic origin, formed after the lava flows cooled down near to the Earth’s surface. In stage II, first an assemblage of calcite (I) and zeolite (I) (including mordenite, heulandite, and stilbite) as well as plagioclase was overgrown by chalcedony, and finally a second calcite (II) and zeolite (II) generation developed by burial metamorphism by subsequent lava flows. Stage III is characterized by precipitation of a third calcite (III) generation together with powellite and apophyllite from late hydrothermal fluids. Rb-Sr and K-Ar ages of apophyllite indicate a large time span for stage III. Apophyllite formed within different time intervals from the Paleogene to the early Miocene even within individual lava flows at certain localities. From the Savda/Jalgaon quarry complex, ages cluster at 44–48 Ma and 25–28 Ma, whereas those from the Nashik area are 55–58 Ma and 21–23 Ma, respectively.

A tourmaline-bearing eclogite from the Kreuzeck Mountains, Eastern Austria, is described in this study as part of an eclogite suite situated SE of the Tauern Window. These rocks formed during Eoalpine high-P metamorphism of the Austroalpine basement. They have high and variable Fe, Ti, P, Zr and Sr contents and unlike other Eoalpine eclogites their Ti–V–Zr–P–Nb–Y systematics indicates a within-plate rather than mid-ocean ridge basalt type environment for the formation of their protoliths. Major and trace element variations are consistent with progressive differentiation of a mafic magma with Fe–Ti oxide and apatite as important igneous phases and with metasomatic addition of K and Sr. The eclogites contain garnet + omphacite + quartz + epidote-group phases + calcic amphibole + K-feldspar ± phengite + rutile + apatite + zircon formed under peak metamorphic P–T conditions of ∼2·1 GPa and ∼650°C. In situ U–Pb dating of two zircons from the tourmaline-bearing sample yields concordant ages of 86 and 109 Ma. Inclusions of K-feldspar + omphacite in garnet, and the very rare presence of phengite as a matrix phase only, indicate very limited availability of fluid during the early stages of the high-P event. Tourmaline formed by fluid influx under eclogite-facies P–T conditions as evidenced by the presence of omphacite and K-feldspar inclusions, and its growth may have continued during uplift and decompression. Tourmaline shows variable and in part extremely high Sr contents of up to ∼5000 µg g⁻¹, which are amongst the highest reported so far for this phase. A possible source of B is the metapelitic country rocks that were pre-enriched in B by the intrusion of tourmaline-bearing pegmatites during a Permian metamorphic event. A crustal source of B is consistent with high Li concentrations in zircons intergrown with the tourmaline. The flow of B–Li–(K–Sr)-bearing fluids into and within the eclogites must have been very localized, as evidenced by the rare occurrence of tourmaline, significant variations in the halogen content of apatite on a thin-section scale and irregular vein-like compositional zoning on a micrometer scale in tourmaline and garnet, which is thought to mark former fluid pathways. Direct evidence for at least two types of fluid present under eclogite-facies P–T conditions is provided by texturally primary H₂O–NaCl–N₂–CH₄–CO₂ fluid inclusions in tourmaline and by H₂O–NaCl–CaCl₂ fluid inclusions in coexisting apatite and quartz. Laser ablation inductively coupled plasma mass spectrometry analysis shows that the primary fluid inclusions in quartz are rich in Ca (1·6–2·7 wt %), K (3500–4900 µg g⁻¹), B (250–370 µg g⁻¹), Sr (490–1080 µg g⁻¹) and Pb (8–16 µg g⁻¹), with total solute concentrations of up to 9·3 wt %. Indirect evidence for the presence of fluids rich in high field strength elements and large ion lithophile elements under amphibolite- and/or greenschist-facies conditions is provided by texturally late light rare earth element–Nb–Ta-rich titanite with a positive Eu-anomaly.

The late Neoproterozoic gabbroic intrusion of the Wadi El-Faliq area in the central Eastern Desert of Egypt (north Arabian–Nubian Shield; henceforth, ANS) is a fresh, undeformed elliptical body elongated in a NW–SE trend following the main sinistral strike-slip faults of the Najd fault system. Mineralogical and geochemical evidence suggest that they were derived from hydrous tholeiitic mafic magmas with arc-like geochemical fingerprints resembling the post-collisional gabbroic intrusions in Saudi Arabia. Despite the arc-like signatures, their fresh and undeformed nature, together with the field relationships, indicates that the studied gabbroic intrusion post-dates the main collisional phase, supporting its emplacement after subduction ceased and during the post-collisional stage. As a result, the arc-like signatures were possibly transmitted from the earlier ANS subduction episode. Indeed, the high (La/Sm)_N, and negative-Nb and positive-Pb anomalies suggest contributions from subduction components. Lithospheric delamination was possibly facilitated by the Najd faults and shear zones formed during the post-orogenic crustal extension associated with the Pan-African orogenic collapse. The delamination process could have generated a rapid upwelling and melting of the asthenosphere mantle. The melt-rock reaction process likely played an important role in the genesis of the studied rocks through the interaction of the asthenosphere melts with lithosphere mantle rocks during ascent. The HREE fractionation suggests a probable mixing between melts from both spinel- and garnet-bearing peridotites. We suggest that the Wadi El-Faliq gabbroic intrusion was likely emplaced due to the stretching and thinning of the lithosphere during the extensional tectonism following the Pan-African orogeny.