scholarly journals Geochemical study of Cenozoic mafic volcanism in the west-central Great Basin, western Nevada, and the Ancestral Cascades Arc, California

Geosphere ◽  
2020 ◽  
Vol 16 (5) ◽  
pp. 1179-1207
Author(s):  
Ann C. Timmermans ◽  
Brian L. Cousens ◽  
Christopher D. Henry
Keyword(s):  
Sierra Nevada ◽  
Great Basin ◽  
Lithospheric Mantle ◽  
Mantle Wedge ◽  
Volcanic Rocks ◽  
Small Subset ◽  
The West ◽  
West Central ◽  
Shallow Subduction ◽  
Mafic Lavas

Abstract Processes linked to shallow subduction, slab rollback, and extension are recorded in the whole-rock major-, trace-element, and Sr, Nd, and Pb isotopic compositions of mafic magmatic rocks in both time and space over southwestern United States. Eocene to Mio-Pliocene volcanic rocks were sampled along a transect across the west-central Great Basin (GB) in Nevada to the Ancestral Cascade Arc (ACA) in the northern Sierra Nevada, California (∼39°–40° latitude), which are interpreted to represent a critical segment of a magmatic sweep that occurred as a result of subduction from east-northeast convergence between the Farallon and North American plates and extension related to the change from a convergent to a transform margin along the western edge of North America. Mafic volcanic rocks from the study area can be spatially divided into three broad regions: GB (5–35 Ma), eastern ACA, and western ACA (2.5–16 Ma). The volcanic products are dominantly calc-alkalic but transition to alkalic toward the east. Great Basin lavas erupted far inland from the continental margin and have higher K, P, Ti, and La/Sm as well as lower (Sr/P)pmn, Th/Rb, and Ba/Nb compared to ACA lavas. Higher Pb isotopic values, combined with lower Ce/Ce* and high Th/Nb ratios in some ACA lavas, are interpreted to come from slab sediment. Mafic lavas from the GB and ACA have overlapping 87Sr/86Sr and 143Nd/144Nd values that are consistent with mantle wedge melts mixing with a subduction-modified lithospheric mantle source. Eastern and western ACA lavas largely overlap in age and elemental and isotopic composition, with the exception of a small subset of lavas from the westernmost ACA region; these lavas show lower 87Sr/86Sr at a given 143Nd/144Nd. Results show that although extension contributes to melting in some regions (e.g., selected lavas in the GB and Pyramid Lake), chemical signatures for most mafic melts are dominated by subduction-related mantle wedge and a lithospheric mantle component.

Lithospheric mantle, asthenosphere, slab and crustal contribution to petrogenesis of Eocene to Miocene volcanic rocks from the west Alborz Magmatic Assemblage, SE Ahar, Iran

2020 ◽  
pp. 1-32
Author(s):  
Ahmad Ahmadvand ◽  
Mohammad Reza Ghorbani ◽  
Mir Ali Asghar Mokhtari ◽  
Yi Chen ◽  
William Amidon ◽  
...  
Keyword(s):  
Lithospheric Mantle ◽  
Mantle Wedge ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
The West ◽  
Nw Iran ◽  
Enriched Mantle ◽  
Basement Rocks ◽  
Cenozoic Volcanism ◽  
Back Arc

Abstract Significant uncertainty remains regarding the exact timing and nature of subduction events during the closure of the Tethyan seas in what is now NW Iran. This study thus presents new geochemical compositions and U–Pb ages for a suite of volcanic rocks emplaced during Cenozoic volcanism in the west Alborz Magmatic Assemblage, which is commonly regarded as the back-arc of the Neotethyan magmatism in Central Iran. The subalkali basalts and andesites are dated to 57 ± 1.2 Ma, and are likely derived from a supra-subduction mantle wedge. Later, trachytic A-type rocks erupted from ~42 to 25 Ma during an anorogenic (extensional) stage triggered by slab retreat and associated asthenospheric mantle influx. A-type melts were at least partly concurrent with lithospheric mantle magmatism implied by eruption of subalkali basalts–andesites around 26–24 Ma. Next, Amp-Bt trachybasaltic volcanism with high-Nb basaltic affinity at ~19 Ma likely records slab deepening and slab partial melting, which reacted with the mantle wedge to produce the source material for the high-Nb basalts. Sr–Nd isotopic ratios for SE Ahar mafic as well as A-type rocks imply rather enriched mantle source(s). Some crustal contamination is implied by the presence of inherited zircons dominated by those derived from Neoproterozoic–Cambrian basement rocks and Carboniferous magmatism. Rhyolitic rocks with adakitic affinity probably mark the final volcanism in the study area. The adakitic rocks show crustal signatures such as high K and Th, probably formed as a consequence of higher temperature gradients, at crustal levels, imposed by both slab and mantle partial melts.

The Intermontane Western Tradition

1962 ◽  
Vol 28 (2) ◽  
pp. 144-150 ◽  
Author(s):  
Richard D. Daugherty
Keyword(s):  
Conceptual Framework ◽  
Sierra Nevada ◽  
Great Basin ◽  
Rocky Mountains ◽  
Environmental Changes ◽  
Glacial Period ◽  
Western Tradition ◽  
The West ◽  
Northern Mexico ◽  
The North

AbstractThe hypothesis of an Intermontane Western tradition is advanced as a conceptual framework within which it is possible to achieve a greater understanding of the cultural histories of the Plateau, Great Basin, and Southwest culture areas, including broad and specific relationships and also the developing differences.Geographically, the Intermontane Western tradition extended throughout the intermontane region between the Cascade-Sierra Nevada ranges on the west, and the Rocky Mountains on the east, and from southern British Columbia on the north to northern Mexico on the south. Temporally, the Intermontane Western tradition existed throughout the post-glacial period.Within the major tradition, the Southwest Agricultural, Desert, and Northwest Riverine Areal traditions are seen developing, partly in response to environmental changes.

Floristic similarity, diversity, and endemism as indicators of refugia characteristics and needs in the West

2020 ◽  
Author(s):  
George P Malanson ◽  
Dale L Zimmerman ◽  
Daniel B Fagre
Keyword(s):  
Sierra Nevada ◽  
Great Basin ◽  
Community Assembly ◽  
Rocky Mountains ◽  
Initial Conditions ◽  
Geographic Distance ◽  
Stepping Stones ◽  
The West ◽  
Floristic Similarity ◽  
Mountain Ranges

The floras of mountain ranges, and their similarity, beta diversity, and endemism, are indicative of processes of community assembly; they are also the initial conditions for coming disassembly and reassembly in response to climate change. As such, these characteristics can inform thinking on refugia. The published floras or approximations for 42 mountain ranges in the three major mountain systems (Sierra-Cascades, Rocky Mountains, and Great Basin ranges) across the western USA and southwestern Canada were analyzed. The similarity is higher among the ranges of the Rockies while equally low among the ranges of the Sierra-Cascades and Great Basin. Mantel correlations of similarity with geographic distance are also higher for the Rocky Mountains. Endemism is relatively high, but is highest in the Sierra-Cascades (due to the Sierra Nevada as the single largest range) and lowest in the Great Basin, where assemblages are allochthonous. These differences indicate that the geologic substrates of the Cascade volcanoes, which are much younger than any others, play a role in addition to geographic isolation in community assembly. The pattern of similarity and endemism indicates that the ranges of the Cascades will not function well as stepping stones and the endemic species that they harbor may need more protection than those of the Rocky Mountains. The geometry of the ranges is complemented by geology in setting the stage for similarity and the potential for refugia across the West. Understanding the geographic template as initial conditions for the future can guide the forecast of refugia and related monitoring or protection efforts.

Superhydrous Arc Magmas in the Alpine Context

Elements ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 35-40 ◽  
Author(s):  
Othmar Müntener ◽  
Peter Ulmer ◽  
Jonathan D. Blundy
Keyword(s):  
Mantle Wedge ◽  
Volcanic Rocks ◽  
Geochemical Data ◽  
The West ◽  
Arc Magmas ◽  
Magmatic Rocks ◽  
Fluid Saturation ◽  
The Alps ◽  
Low K ◽  
Primary Magmas

Magmatic rocks in the Alps are scarce. What little arc magmatism there was pre-dates the Eurasia–Adria collision at 43–34 Ma but ends at 30–29 Ma. Conversely, geochemical data for magmatic rocks from the Alps resemble that of subduction-related magmatic arcs. A characteristic of Alpine magmatism is the occurrence of relatively deep (80–100 km) super-hydrous (>8 wt% H2O) low-K primary magmas in the east and shoshonitic K-rich magmas in the west. These features are likely related to the absence of vigorous mantle wedge convection. Superhydrous primary magmas undergo extensive crystallization and fluid saturation at depth, producing high ratios of plutonic to volcanic rocks. We speculate that superhydrous primary arc magmas are a consequence of slow convergence and the initial architecture of subducting crust.

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