Detailed petrography, microstructure, and geochemistry of garnet pyroxenite xenoliths in Holocene basanite tuffs from maars at Lakes Bullenmerri and Gnotuk (western Victoria, southeastern Australia) have been used to track their igneous and metamorphic history, enabling the reconstruction of the thermal-tectonic evolution of the lithospheric mantle. The exsolution of orthopyroxene and garnet and rare spinel, plagioclase, and ilmenite from complex clinopyroxene megacrysts suggests that the xenoliths originally were clinopyroxene-dominant cumulates associated with minor garnet, orthopyroxene, or spinel. The compositions of exsolved phases and their host clinopyroxene were reintegrated using measured modal proportions to show that the primary clinopyroxene was enriched in Al2O3 (5.53–13.63 wt%) and crystallized at ~1300–1500 °C and 16–30 kbar. These cumulates then underwent extensive exsolution, recrystallization, and reaction during cooling, and finally equilibrated at ~950–1100 °C and 12–18 kbar before entrainment in the basanites. Rare earth element (REE) thermobarometry of garnets and coexisting clinopyroxenes preserves evidence of an intermediate stage (1032 °C and 21 kbar). These results imply that the protoliths of the garnet pyroxenite formed at a range of depths from ~50 to 100 km, and then during or shortly after cooling, they were tectonically emplaced to higher levels (~40–60 km; i.e., uplifted by at least 10–20 km) along the prevailing geotherm. This uplift may have been connected with lithosphere-scale faulting during the Paleozoic orogeny, or during Mesozoic–Cenozoic rifting of eastern Australia.

Integration of geochronological analyses of in situ zircons with the petrological study of their microstructural growth microdomains (through petrography and phase diagram modeling) yields a more precise interpretation and understanding of any geological process than the study of ages from separate zircons. This is essential especially for those rocks affected by polymetamorphic histories. We present new in situ U-Pb zircon dating from metapelitic granulites exposed in the contact with the ultramafic massif of Beni Bousera (northern Morocco, Western Mediterranean). Geochronological data scatter from Paleoproterozoic (1508 ± 23 Ma) to Miocene (22.9 ± 0.7 Ma), though the majority of ages range between 100 and 400 Ma, with four main peak ages clustering around 390–319, 286–264, 193, and 105 Ma. The Middle Permian (ca. 286–264 Ma) and earliest Miocene (22.9 ± 0.7 Ma) ages are constrained from a thermobarometric point of view: The former ages represent the formation of the zircons at depth (higher pressure), and the latter ages represent the exhumation process (lower pressure). Combining these petrological results with previous geophysical and geodynamical studies from the literature in the area, we propose a four-step geodynamic model for the Beni Bousera orogenic peridotite since the end of the Paleozoic (i.e., Middle Permian) to the present, supporting an evolution linked to a subduction-related scenario since Late Cretaceous times.

Based on relationships among volcanic-plutonic arc rocks, high-pressure–low-temperature (HP-LT) metamafic rocks, westward relative migration of the Klamath Mountains salient, and locations of the Mariposa-Galice, Great Valley Group, and Franciscan depositional basins, the following geologic evolution is inferred for the northern California continental edge: (1) By ca. 175 Ma, onset of transpressive plate underflow generated an Andean-type Klamath-Sierran arc along the margin. At ca. 165 Ma and continuing to ca. 150–140 Ma, erosion supplied volcanogenic debris to proximal Mariposa-Galice ± Myrtle overlap strata. (2) Oceanic crustal rocks were metamorphosed under HP-LT conditions in an inboard, east-inclined subduction zone from ca. 165 to 150 Ma. Most such mafic rocks remained stored at depth, and HP-LT tectonic blocks only returned surfaceward during the Late Cretaceous, chiefly entrained in circulating, buoyant Franciscan mud-matrix mélange. (3) At end-of-Jurassic time, before onset of paired Franciscan and Great Valley Group + Hornbrook deposition, the Klamath salient was deformed and displaced ∼100–200 km westward relative to the Sierran arc. (4) After this ca. 140 Ma seaward step-out of the Farallon–North American convergent plate junction—stranding preexisting oceanic crust on the south as the Coast Range ophiolite—terrigenous debris began to arrive at the Franciscan trench and intervening Great Valley forearc. Voluminous sedimentation and accretion of Franciscan Eastern + Central belt and Great Valley Group coeval detritus took place during paroxysmal igneous activity and rapid, nearly orthogonal plate convergence at ca. 125–80 Ma. (5) Sierran arc volcanism-plutonism ceased by ca. 80 Ma in northern California, signaling a transition to shallow, nearly subhorizontal eastward plate underflow attending Laramide orogeny far to the east. (6) Paleogene–Lower Miocene Franciscan Coastal belt sedimentary strata were deposited in a tectonic realm nearly unaffected by HP-LT subduction. (7) Grenville-age detrital zircons apparently are absent from the post–120 Ma Franciscan section. Detritus from the Pacific Northwest is not present in the Central belt sandstones, whereas zircons from the Idaho Batholith, the Challis volcanics, and the Cascade Range appear in progressively younger Paleogene–Lower Miocene Coastal belt sediments. This trend suggests the possible gradual NW dextral offset of Franciscan trench deposits of up to ∼1600 km relative to the autochthonous Great Valley Group forearc and basement terranes of the American Southwest.

The association of Hawaiian-Emperor volcanism with a large-scale central Pacific anisotropy anomaly at ~150 km depth can be explained by tapping of shallow melt sources in a perisphere/LLAMA (layer of lateral advection of mass and anisotropy) model. The origin of the anisotropy anomaly can be traced to the formation of a phlogopite-garnet-pyroxenite assemblage in the perisphere beneath an island arc on the Stikine terrane of the North American Cordillera in the Carboniferous. The pyroxenites were formed when subduction-related melts invaded the mantle wedge at ~150–200 km depth. The enriched region inherited the thermal profile of the mantle wedge, along with a solar-like noble gas isotopic composition from earlier fluxing of hydrothermal fluids between interplanetary dust particle–bearing deep-sea sediments and ultramafic layers of the oceanic crust prior to subduction. After termination of subduction, the enriched perisphere was displaced to the northeast beneath the Farallon plate, and then to the northwest beneath the Izanagi and Pacific plates, eventually becoming distorted into the shape of the present-day central Pacific anisotropy anomaly. During the thermal equilibration time, estimated at ~170 m.y., the phlogopite-garnet-pyroxenite assemblage followed a horizontal trajectory in pressure-temperature (P-T) space. As the P-T path crossed the solidi for volatile-bearing pyroxenite compositions, diabatic partial melting generated carbonatitic to alkaline melts which began to ascend and metasomatize shallower levels of the perisphere, carrying with them the geochemical signature of the original pyroxenites. The present central Pacific anisotropy anomaly is the current manifestation of the metasomatized domain. The latter was tapped from the Late Cretaceous to the present, by propagating fractures induced by large-scale plate reorganizations in the northwest of the Pacific Basin, to produce the Hawaiian-Emperor volcanic chain.

The Siberian craton was affected by flood basalt volcanism at least twice during the Devonian (Yakutsk-Vilyui province) and Permian–Triassic (Siberian province) periods. In both cases volcanism appeared as brief pulses of flood basalt eruptions, followed by kimberlitic (and lamproitic) emplacement. Pressure estimations for the kimberlite-entrained mantle xenoliths reflect that the lithosphere was 190–230 km thick at the time of the Devonian flood basalt volcanism. Differently from Devonian kimberlites, the majority of Triassic kimberlites are diamond free, but at least one Triassic kimberlite pipe and some lamproites are diamondiferous, suggesting that the Siberian lithosphere remained thick during the Permian–Triassic flood basalt volcanic activity. If both the lithosphere and the asthenosphere were volatile poor, thick cratonic lithosphere prevented melting even at an elevated geotherm. During the Paleozoic, Siberia was surrounded by subduction systems. The water deep cycle in association with fast subduction and slab stagnation in the mantle transition zone is proposed to cause fluxing of the asthenosphere by water plus other fluids via wet diapir formation in the mantle transition zone. Such diapirs started to melt in the asthenosphere beneath thick cratonic lithosphere, producing voluminous melts. Mafic melts probably accumulated beneath cratonic lithosphere and rapidly erupted on the surface in response to stress-induced drainage events, as assumed for some other cratonic flood basalts.

Extract from beginning of chapter: VOLCANIC CORES The discovery of this center of volcanic eruption, the core of what has since been called the dissected volcano of Crandall Basin (Iddings, 1893b), was one of those delightful experiences which happen occasionally in the lifetime of an explorer in any field of research. It was more delightful than the finding of the volcano core at Electric Peak had been two years before, because that had been anticipated in a measure by the discovery of a specimen of diorite in Wright's collection with its suggestion of what was to follow. The finding of the Crandall core was the outcome of several days' exploration of the volcanic ridges south, where dikes of andesite-porphyry and basalt were observed trending from various places toward one spot beyond a narrow steep ridge on the south side of Crandall Creek. On the crest of this narrow ridge dikes were very numerous and the focus of their directions very definite, so that we had great hopes that there might be a core of coarse-grained rocks exposed to view at no great distance. Camp was accordingly moved across the narrow ridge into the deep valley of Crandall Creek. The descent into the valley was a scramble for horses and pack mules, down the steep slope through small timber, and it was necessary to locate camp up the valley for grass and level ground. Not until the next morning could I explore the lower, narrow valley for the expected coarse-grained rocks. They appeared where they were

The basement of the East European Platform corresponding to northeastern Sarmatia is known as the Voronezh Crystalline Massif (VCM). The Kursk microcontinent, which lies in northeastern Sarmatia, occupies the bulk of the Voronezh Crystalline Massif. The predominant portion of the Kursk microcontinent is a combination of sedimentary–volcanic complexes making up greenstone belts and granite-gneiss (granite-migmatite) associations of the granite-greenstone domain bearing the same name. The smaller Kursk–Besedino granulite-gneiss terrane is situated in the central part of the microcontinent. The following sequence of events may be proposed as a preliminary model of crustal evolution: (1) Paleo- to Mesoarchean: formation of granite-greenstone continental crust (3.7–3.1 Ga); (2) events related to the activity of a mantle plume 2.85–2.82 Ga ago: underplating by mantle-derived magmas; formation of an intracontinental depression; its rapid filling with sediments, including Fe-rich varieties; and metamorphism of granite-greenstone basement and the sedimentary fill of the depression; and (3) Neoarchean and/or Paleoproterozoic: collisional compression and transformation of the depression into a synformal tectonic nappe.

The major tectonic units of the Neoarchean Volgo-Uralia continent, which is ~600,000 km2 in area, are contrastingly expressed in regional gravity and magnetic maps. Interpretations of seismic images of the crust along the TATSEIS geotraverse in combination with 3D density and magnetic crust models provide insights into the volumetric representation of tectonic structures of various ranks. Granulite-gneiss crust of Volgo-Uralia is characterized by elevated thickness (~60 km and locally up to 65–70 km). The deep structure of Volgo-Uralia assumes that the entire crustal section, including the lower crust, is composed of high-density granulite metamorphic facies rocks. Specific structural units called ovoids play the main role in the structure of this continent. The ovoids are bowl-shaped crustal blocks, round or oval in plan view, 300–600 km across, and with the base reaching a level of crust-mantle interface at ~60 km. Ovoids are bounded by conic surfaces of reverse (thrust)-faults, along which their outer parts are thrust over the framework. The Tokmov, Buzuluk, Verkhnekamsk, Krasnoufimsk, and Orenburg ovoids, which generally are not in contact with one another, are dominated by mafic granulites, gabbroic rocks, gabbroanorthosites, and ultramafic rocks. A significant contribution of deep-seated intrusive rocks suggests that metamorphism developed in the lower and middle crust at high PT parameters that exceed the maximum estimates (940–950 °C, 9.5 kbar) recorded in samples of borehole cores. The interovoidal space is occupied by elongated oval synforms up to 200–400 km long. This space is considered to be an interovoidal domain, which includes three relatively narrow, compressed synforms (Yelabuga-Bondyug, Kilmez, Chusovaya) and four oval synforms (Srednevyatka, Verkhnevyatka, North-Tatar, Almetevsk). The Tokmov ovoid is framed in the southeast by the Tuma and Penza belts. Synforms are filled with metasedimentary granulites and mafic metaigneous rocks. The protoliths were formed over the time interval from 3.4–3.2 to 3.1–3.0 Ga. The internal zoning of the Volgo-Uralia crust is related to numerous local centers within ovoids and interovoidal region. At least two high-temperature metamorphic events were followed by periods of retrogression: 2.74–2.70 and 2.62–2.59 Ga. The areal and especially high-temperature character of tectonothermal processes during formation of the Neoarchean crust of the Volgo-Uralia Craton, and distinct geometrization of space with recognition of several concentric domains, finds a universal explanation in ascent of multiple plumes.