Lithostratigraphy and lithogeochemistry of Ediacaran alkaline basaltic rocks of the Musgravetown Group, Bonavista Peninsula, northeastern Newfoundland, Canada: an extensional volcanogenic basin in the type-Avalon terrane

Volcanic rocks of the Ediacaran Musgravetown Group on Bonavista Peninsula, Avalon terrane, Newfoundland, include basal ca. 600 Ma calc-alkaline basalt succeeded by continental tholeiite and alkaline rhyolite of the ca. 592 Ma Plate Cove volcanic belt (Bull Arm Formation), indicating a change from subduction-related to extensionrelated tectonic regimes during that interval. Alkalic basalts on northeastern (Dam Pond area) and southwestern (British Harbour area) Bonavista Peninsula occur below and above, respectively, the ca. 580 Ma glacial Trinity facies. Dam Pond basalt occurs in a structural dome intercalated with and flanked by fine-grained, siliciclastic deposits (Big Head Formation) overlain by Trinity facies. The British Harbour basalt occurs above the Trinity facies, in an upward- coarsening sandstone sequence (Rocky Harbour Formation) overlain by red beds of the Crown Hill Formation (uppermost Musgravetown Group). The Rocky Harbour and Big Head formations are likely stratigraphically interfingered proximal and distal deposits, respectively, derived from erosion of the Bull Arm Formation and older Avalonian assemblages.The Big Head basalts have lower SiO2, Zr, FeOT, P2O5, TiO2 and higher Mg#, Cr, V, Co and Ni contents, and are therefore more primitive than the more FeOT-, TiO2-, and P2O5-rich British Harbour basalts. Large-ionlithophile and rare-earth-element concentrations and ratios indicate that both suites originated from low degree partial melts of deep, weakly garnet-bearing, undepleted asthenospheric peridotite sources, with magma conduits likely focused along regional extensional faults. The protracted and episodic extension-related volcanic activity is consistent with a geodynamic setting that evolved from a mature arc into extensional basins with slowly waning magmatism, possibly involving slab rollback and delamination followed by magmatic underplating. The duration and variation of both volcanism and sedimentation indicate that the Musgravetown Group should be elevated to a Supergroup in order to facilitate future correlation of its constituent parts with other Avalonian basins.

Exploring rift magmatism and evolution through gravity analysis of North America's failed rifts

<p>Comparative study of North America’s failed continental rifts allows investigation of the effects of extension, magmatism, magmatic underplating and rift inversion in the evolution of rifting. We explore this issue by examining the gravity signatures of the Midcontinent Rift (MCR), Reelfoot Rift (RR), and Southern Oklahoman Aulacogen (SOA). The ~1.1 Ga MCR records aspects of the complex assembly of Rodina, while the structures related to the ~560 Ma RR and SOA formed during the later breakup of Rodinia and subsequent assembly of Pangea. Combining average gravity anomalies along each rift with seismic data, we examine whether these data support the existence of high-density residual melt underplates (“rift pillows”), reflect the possible amounts of inversion, and whether these rifts should be considered analogs of one another at different stages in rift evolution. The MCR and SOA have strong gravity highs along much of their length. Furthermore, the west and east arms of the MCR have different gravity signatures. The west arm of the MCR has a positive gravity anomaly of 80-100 mgals, while the east arm and SOA have positive anomalies of only 40-50 mgals. The RR does not exhibit a high positive anomaly along much of its length. The positive anomalies of both arms of the MCR and SOA reflect 10-20 km thick underplates at the base of the crust. These gravity anomalies also reflect greater amounts of inversion, during which the rift-bounding normal faults are reactivated by later compression, bringing the high-density igneous rocks closer to the surface. By averaging gravity data along the length of each failed rift, we can more easily distinguish between the history of individual rifts and general features of rifting that apply to other failed or active rifts around the world.</p>

Gravity modeling of the Alpine lithosphere affected by magmatism based on seismic tomography

The southern Alpine regions were affected by several magmatic and volcanic events between the Paleozoic and the Tertiary. This activity undoubtedly had an important effect on the density distribution and structural setting at lithosphere scale. Here the gravity field has been used to create a 3D lithosphere density model on the basis of a high-resolution seismic tomography model. The results of the gravity modeling demonstrate a highly complex density distribution in good agreement with the different geological domains of the Alpine area represented by the European Plate, the Adriatic Plate and the Tyrrhenian basin. The Adriatic-derived terrains (Southalpine and Austroalpine) of the Alps are typically denser (2850 kg m−3), whilst the Alpine zone, composed of terrains of European provenance (Helvetic and Tauern Window), presents lower density values (2750 kg m−3). Inside the Southalpine, south of the Dolomites, a well-known positive gravity anomaly is present, and one of the aims of this work was to investigate the source of this anomaly that has not yet been explained. The modeled density suggests that the anomaly is related to two different sources; the first involves the middle crust below the gravity anomaly and is represented by localized mushroom-shaped bodies interpreted as magmatic intrusions, while a second wider density anomaly affects the lower crust of the southern Alpine realm and could correspond to a mafic and ultramafic magmatic underplating (gabbros and related cumulates) developed during Paleozoic extension.

Mechanism for epeirogenic uplift of the Archean Dharwar craton, southern India as evidenced by orthogonal seismic reflection profiles

Plateaus, located far away from the plate boundaries, play an important role in understanding the deep-rooted geological processes responsible for the epeirogenic uplift and dynamics of the plate interior. The Karnataka plateau located in the Dharwar craton, southern India, is a classic example for the plateau uplift. It is explored using orthogonal deep crustal seismic reflection studies, and a mechanism for the epeirogenic uplift is suggested. A pseudo three-dimensional crustal structure derived from these studies suggests a regionally extensive 10 km thick magmatic underplating in the region. It is further constrained from active-source refraction and passive-source seismological data. We interpret the Marion and Reunion mantle plume activities during 88 Ma and 65 Ma on the western part of Dharwar craton are responsible for the magmatic underplating, which caused epeirogenic uplift. Flexural isostasy related to the onshore denudational unloading and offshore sediment loading is also responsible for the persisting uplift in the region. Plate boundary forces are found to be contributing to the plateau uplift. The present study provides a relationship between the mantle plumes, rifting, development of continental margins, plateau uplift, and denudational isostasy. Combination of exogenic and endogenic processes are responsible for the plateau uplift in the region.

Petrology of the April 2015 Eruption of Calbuco Volcano, Southern Chile

Understanding the origin of intermediate magmas that commonly erupt from subduction zone volcanoes is important to better constraining the mechanisms of continental crust formation. We performed a detailed mineralogical and petrological study of the eruptive products from the April 2015 eruption of Calbuco volcano, Chile, a three-phase sub-Plinian eruption that produced pyroclastic deposits of andesitic composition. The eruptive products comprise a glass phase and a high but variable proportion of minerals dominated by plagioclase, clinopyroxene, and orthopyroxene, with minor olivine, amphibole, and magnetite. Plagioclase is very strongly zoned with highly anorthitic cores surrounded by more albitic rims, and no intermediate compositions between them. Based on thermodynamic calculations and published experimental data, we estimate that the anorthitic cores crystallized from a basaltic andesite melt containing 3·5–4·5 wt% H2O. The bulk-rock major and trace element variability at Calbuco is best explained by the accumulation of a variable amount of minerals (in relative proportion 72 % plagioclase, 28 % pyroxene) in a dacitic melt. These minerals most probably formed in the crystal mush zone of the magma chamber, at 200–300 MPa (8–11 km depth) according to pyroxene and amphibole compositions. A few weeks to months before the eruption, the crystal mush was disaggregated, perhaps owing to magmatic underplating, and a crystal-bearing dacitic melt migrated into a subsurface storage region where the albitic plagioclase rims crystallized. The eruption was probably internally triggered by over-pressurization in the shallow magma chamber.

Gravity modeling of the Alpine lithosphere affected by magmatism based on seismic tomography

The Southern Alpine regions have been affected by several magmatic and volcanic events between the Paleozoic and the Tertiary. This activity has undoubtedly had an important effect on the density distribution and structural setting at lithosphere scale. Here the gravity field is used to create a 3D lithosphere density model on the base of a high-resolution seismic tomography model. The results of the gravity modelling demonstrate a highly complex density distribution in good agreement with the different geological domains of the Alpine area represented by the European plate, the Adriatic plate and the Tyrrhenian basin. The Adriatic derived terrains (Southalpine and Austrolpine) of the Alps are typically denser (2850 kg m−3), whilst the Alpine zone composed by European terrains provenance (Helvetic and Tauern window) presents lower density values (2750 kg m−3). Inside the Southalpine, south of the Dolomites, a well-known positive gravity anomaly is present and one of the aims of this work is to investigate the source of this anomaly that has not yet been explained. The modelled density suggests that the anomaly is related to two different sources, the first involves the middle-crust below the gravity anomaly and is represented by localized mushroom-shaped bodies interpreted as a magmatic intrusion, while a second wider density anomaly affects the lower crust of the Southern Alpine realm and could correspond to a mafic and ultramafic magmatic underplating (gabbros and related cumulates) developed during Paleozoic extension.