Extension-related Magmatism Research Articles

Granites and the geodynamic history of the neoproterozoic Brası́lia belt, Central Brazil: a review

Recent field and geochronological studies have demonstrated the importance of granitic magmatism in the evolution of the Neoproterozoic Brası́lia Belt, in Central Brazil. This is an orogenic belt developed in response to the convergence between the Amazon, São Francisco–Congo and Paraná continental blocks. The presence of Neoproterozoic juvenile arc rocks and syn-collisional peraluminous granites challenged previous intracontinental evolution models for the belt. The granitoid intrusions reviewed in this paper record the different stages of evolution of the orogen and their field and isotopic characteristics can be used to reconstruct the tectonic history of the belt. The main field and isotopic characteristics of four granite suites associated with the Brası́lia Belt are reviewed: (i) 1.77–1.58 Ga old rift related A-type granite intrusions, (ii) ca. 0.8–0.7 syn-collisional granitoids, (iii) arc metatonalites and metagranodiorites (ca. 0.9 to 0.63 Ga), and (iv) bimodal post-orogenic suite ranging in age from ca. 0.59 to 0.48 Ga. These rocks suggest that during most of the Neoproterozoic the western margin of the São Francisco continent faced a large oceanic basin, where subduction and oceanic lithosphere consumption started at ca. 0.9 Ga, roughly coeval with the initial stages of the break up of Rodinia. Final ocean closure happened at ca. 0.63–0.60 Ga with crustal thickening, uplift and erosion. Post-orogenic extension-related magmatism took place between ca. 0.6 and 0.5 Ga and was partially contemporaneous with the deposition of the Paraguay and Tucavaca sedimentary successions, resulting from the rifting event related to the break up of Laurentia from southwestern Gondwana.

Origin and implications of mafic xenoliths associated with Cenozoic extension-related volcanism in the V�lencia Trough, NE Spain

The Valencia Trough of northeastern Spain represents a poorly understood region of Cenozoic extension-related magmatism in western Europe. We present chemical data on mafic xenoliths and their host basalt from the Roca Negra cinder cone in the Catalan Volcanic Zone in the Valencia Trough. The xenoliths consist of augite+hornblende+ oxides±plagioclase±apatite±olivine::Lbiotitelorthopyroxene, and range from clinopyroxenites and hornblendites to gabbroic rocks. The major and trace element compositions of the xenoliths are compatible with formation as cumulates from a range of olivine tholeiitic to strongly alkaline magmas, and show a strong affinity to the Middle Miocene to Recent rift-related magmatism in the Catalan Volcanic Zone. The xenoliths formed at temperatures of 1110–1180 °C, pressures of 2–7 kb (with 10 kb as the upper limit), and oxygen fugacities corresponding to 1–2 log units above the NNO oxygen buffer. The estimated pressure range implies formation in magma chambers within the crust, possibly also near the crust/mantle boundary (underplating). The presence of crustal cumulates indicates that the total volume of magmas emplaced into the crust in the Catalan Volcanic Zone may be significantly larger than indicated by surface exposures. Isotopic compositions of the xenoliths are87Sr/86Sr: 0.703537–0.703647,143Nd/144Nd: 0.512732–0.512766,206Pb/204Pb: 18.097–19.106,207Pb/204Pb: 15.522–15.579, and208Pb/204Pb: 37.850–38.794. This range suggests the involvement of two mantle source components. One of these may be the low-velocity component recognized in Cenozoic magmatism in the eastern Atlantic Ocean to central Europe and western Mediterranean. The other component, which differs from those involved in other domains of Cenozoic volcanism in Europe and adjacent areas, may be located in the lithospheric mantle underlying NE Spain, which was metasomatized during late Variscan to early Alpine deformation.

The nature of Triassic extension-related magmatism in Greece: evidence from Nd and Pb isotope geochemistry

The widespread Triassic volcanic rocks of Greece, dismembered during the Hellenide orogeny, are used to interpret the nature of Triassic rifting. Four assemblages of volcanic rocks are distinguished on geochemical criteria: (1) a predominant subalkaline basalt–andesite–dacite series with a high proportion of pyroclastic rocks; (2) minor shoshonites; (3) alkali basalt and (4) MORB. The stratigraphic and palaeogeographic distribution of these rock types is synthesized. New Pb and Nd isotopic data are used to discriminate between hypotheses suggesting that either subduction or extension was responsible for the Triassic volcanism. In the subalkaline basalt assemblage, εNd is negative with depleted mantle model ages >1.5 Ga. Pb isotopic compositions are mostly close to the very distinctive compositional field of Cenozoic extensional rocks of the Aegean area, with very high 207Pb/204Pb for relatively low 206Pb/204Pb ratios. These isotopic data confirm interpretations based on trace elements that subalkaline basalts were predominantly derived from melt-depleted peridotite in the sub-continental lithospheric mantle as a result of extension. Small areas of enriched hydrous mantle partially melted to yield shoshonitic magmas. Nd and Pb isotopic compositions of the alkali basalts are quite different from those in other rock types and suggest a HIMU mantle source component derived from a small plume, which also influenced MORB compositions. Distribution of these various rock types is used to constrain palaeogeographic reconstruction of Triassic micro-continental blocks.

U–Pb age of the Seal Lake Group, Labrador: relationship to Mesoproterozoic extension-related magmatism of Laurasia

The Seal Lake Group of eastern Labrador unconformably overlies the 1273 ± 1 Ma (2σ) Harp Dyke Swarm. In its lower parts, the Seal Lake Group includes conglomerates, quartzites, and red-bed sediments, and both the lower and middle parts are intruded by olivine gabbro sills. This magmatism was coeval with the emplacement of basaltic volcanic rocks in the middle and upper sections of the Seal Lake Group. Two different gabbro varieties yield baddeleyite and zircon U–Pb ages of [Formula: see text] Ma and [Formula: see text] Ma, respectively. The older age corresponds to the emplacement of a fine- to medium-grained voluminous olivine gabbro, whereas the younger age was obtained on a coarse-grained gabbro that intrudes the fine-grained gabbro. These ages place magmatism in the Seal Lake area in the ca. 1270–1220 Ma phase of extension-related magmatism of Laurentia and Baltica that was associated with voluminous mafic dyke swarms. Such extension-related mafic Mesoproterozoic magmatism occurs on both cratons during three distinct intervals at 1460–1420, 1270–1220, and 1180–1070 Ma, respectively, suggesting that Laurentia and Baltica formed a coherent unit until at least 1180 Ma.

A Binary Source Model for Extension-Related Magmatism in the Great Basin, Western North America

Models for extension-related magmatism based on decompression melting of asthenospheric mantle poorly simulate fluxes and bulk compositions of magmas produced during early stages of continental extension. For the Great Basin of western North America, it is proposed that magmatism proceeded in two stages, the first involving melting of lithospheric mantle sources between 40 and approximately 5 million years ago (Ma), followed (since approximately 5 Ma) by melting of upwelling asthenospheric mantle in areas where extension has exceeded about 100 percent. This transition in magma sources is diachronous, depending on initial variations in lithosphere thickness and on rates of lithospheric thinning.

Early Miocene continental extension-related basaltic magmatism at Walton Peak, northwest Colorado: further evidence on continental basalt genesis

The Walton Peak lavas erupted directly onto c. 1.8 Ga basement and were interbedded with subaerial sediments. Four new K-Ar dates for the lavas average 22.8 ± 0.3 Ma, associating them unambiguously with the earliest large-scale extension-related magmatism in NW Colorado. The lavas are basalts, trachybasalts and shoshonites. There is also one 100 m thick composite flow of trachydacite containing pillow-like basic masses (up to tens of metres in size) with chilled margins. Elemental data and Sr and Nd isotopic ratios suggest that the trachydacite evolved from the basalt type forming most of the pillows, by a combination of fractional crystallization and crustal assimilation. Nevertheless, post-evolution magma mixing, during and immediately before extrusion of the flow, has obscured the details of this process. A proportion of the pillow-like basic masses in the composite flow are shoshonites that did not take part in the magma mixing. They are relatively rich in incompatible minor and trace elements; e.g. Nb = 41–46 ppm in the shoshonites and 20–25 ppm in all other Walton Peak basic lavas. The Sr and Nd isotopic ratios of the Nb-rich and Nb-poor compositions overlap. Open-system processes in a long-lived pre-existing magma chamber are considered unlikely to be the main cause of the diverse basic magmas because the Walton Peak lavas directly overlie Precambrian basement. Likewise, crustal contamination models using either upper or lower crustal rock types of this region do not give satisfactory mechanisms to relate the Nb-rich and Nb-poor mafic liquids. A genetic model invoking variable degrees of partial melting of lithospheric mantle containing hydrous minerals does not explain why the Nb-rich and Nb-poor compositions are bimodal and not a range. A variety of sparse ultrapotassic magmas (minettes to lamproites) accompanied the basalt-dominated Neogene volcanism of NW Colorado and surrounding area. Their elemental and isotopic compositions support the view that the ultrapotassic liquids originated by fusion of hydrous and halogen-rich metasomatized zones within the subcontinental lithospheric mantle. Addition of about 15% of lamproitic melt to the low-Nb Walton Peak basic magmas reproduces the main geochemical and isotopic characteristics of the high-Nb magmas. Such a process seems to have been widespread throughout NW Colorado Neogene igneous activity. Although wholly lithospheric mantle sources for all the mafic magmas cannot be ruled out, there is evidence that both subduction-related calcalkaline and ocean-island basanitic melts of asthenospheric origin formed the dominant components of the NW Colorado volcanics. Addition of as little as 10% of lamproitic melt, during their uprise through the subcontinental lithospheric mantle, was sufficient to imprint on them ‘continental’ incompatible element and radiogenic isotope characteristics.

Homogenization and lowering of 18O/ 16O in mid-crustal rocks during extension-related magmatism in eastern Nevada

The oxygen isotope systematics of Tertiary volcanic rocks of east-central Nevada and of plutonic and metamorphic rocks of the Ruby Mountains-East Humboldt Range core complex provide complementary evidence for major 18O-depletion and 18O/ 16O homogenization of mid-crustal rocks during metamorphism and magmatism. The δ 18O value of crustal source material for silicic volcanic rocks decreased from between +9 and +11‰ to between +7 and +8‰ over ∼ 5 Ma. Mid-crustal metasedimentary and granitic rocks in the East Humboldt Range have δ 18O values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels. Exchange with deep-seated mantle-derived igneous rocks, or fluids derived therefrom, is the most plausible 18O-depletion mechanism. Intrusion of these mafic magmas promoted crustal melting and fluid migration. Homogenization of 18O/ 16O resulted from migration of high-temperature fluids and melts at mid-crustal levels, and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks.

Early proterozoic geology of the great lakes region

The Belleview metadiabase sill crosscuts the Siamo Slate and the Negaunee Iron Formation of the Marquette Range Supergroup, Michigan. It is one of a series of metamorphosed diabase sills that intrudes the Menominee Group and possibly the Baraga Group. Chloritic sills within the Marquette Range Supergroup have yet to be officially grouped as one or more series or swarms and therefore stratigraphic correlations and temporal relationships between these sills remain speculative. The recovery of the first sensitive high-resolution ion mass spectrometry (ID-TIMS) U–Pb date of 6 baddeleyite grains from the Belleview sill yielding an age of 1891 ± 3 Ma is a positive step to classifying these chloritic metadiabases. This new date has three important implications: (1) it gives a definitive minimum age of Negaunee Iron Formation deposition; (2) it confirms recent stratigraphic correlations that suggest that the Negaunee Iron Formation is older than the Mesabi, Gunflint and Gogebic Iron Ranges and not contemporaneous; and (3) it does not support recent correlations of the Menominee Group chloritic sills with the 1874 Ma Hemlock Formation within the Baraga Group of the Marquette Range Supergroup.This new date does not support a more recently proposed synorogenic foredeep depositional environment for the deposition of the Negaunee Iron Formation. Instead, this age date supports an earlier interpretation that the Negaunee Iron Formation is part of continental shelf deposits laid down prior to onset of 1875–1835 Ma collision. The age of the Belleview sill suggests that it is the result of extension-related magmatism that is prior to the currently defined Penokean orogenic event.