Proterozoic greenstone associated with the Pinal Schist in the Little Dragoon Mountains, southeastern Arizona: Episodic mafic magmatism and tectonism in the Mazatzal Province of southwestern Laurentia

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Author
Thorkelson, Derek J.
Carter, Maria R.
Johnson, Bradford J.
Donald, Roberta L.
Affiliation
Department of Earth Sciences, Simon Fraser University, Burnaby BC, Canada
Arizona Geological Survey, University of Arizona, Tucson, AZ
Issue Date
2025-04
Keywords
Proterozoic
greenstone
Arizona
Mazatzal Province
magmatism
tectonism

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Citation
Thorkelson, D.J., Carter, M.R., Johnson, B.J. and Donald, R.L., 2025. Proterozoic greenstone associated with the Pinal Schist in the Little Dragoon Mountains, southeastern Arizona: Episodic mafic magmatism and tectonism in the Mazatzal Province of southwestern Laurentia. Arizona Geological Survey, Open File Report OFR-25-3, 52 p.
Publisher
Arizona Geological Survey (Tucson, AZ)
Description
The Pinal Schist and associated metavolcanic and plutonic units are major constituents of the late Paleoproterozoic Mazatzal orogenic province in southeastern Arizona and adjacent parts of New Mexico and Sonora. These rocks are well exposed in the Little Dragoon Mountains of southeastern Arizona, where new geological mapping and sampling was concentrated in the Sheep Basin and Seven Dash Ranch areas. There, the Pinal Schist hosts bodies of Pinal Greenstone, i.e., mafic igneous rocks metamorphosed to lower greenschist grade. Field relations, along with major and trace element geochemistry, and Nd isotope geochemistry, were used to divide the greenstones into four categories, termed Types A, B, C and D. Isotopic age determinations were not obtained but approximate numerical ages of the four greenstone types were assigned on the basis of local field relations such as intensity of foliation and crosscutting relations, and constraints from previously published Mazatzal igneous and sedimentary rocks in Arizona and New Mexico. Greenstone Type A is strongly foliated and has a composition similar to modern, slightly enriched mid-ocean ridge basalt, an assigned age of 1.68 Ga, and ϵNd(t) from 3.5-5.2. Type B is less foliated, has a mid-ocean ridge basalt composition, an assigned age of 1.67 Ga, and ϵNd(t) from 4.1-4.6. Type D is similar in appearance to Type B, but has a composition similar to primary basalt in modern island arcs, an assigned age of 1.64 Ga, and ϵNd(t) from 2.0-3.8. Types A, B and D have Nd depleted mantle model ages (Tdm) from 1.8-2.6 Ga. Type C is a pyroxenite (interpreted as an ultramafic cumulate) with volcanic arc affinities. The age of Type C is poorly constrained, with Tdm ages from 1.1- 1.5 and steep lines of isotopic evolution. For the purpose of calculating ϵNd(t), Type C has been assigned a nominal age of 1.1 Ga, yielding ϵNd(t) from 3.9-7.2. Collectively, this information is consistent with derivation of the Pinal Greenstone in our study area from slightly enriched mantle, with Nd isotopic compositions intermediate between depleted mantle and chondritic uniform reservoir (CHUR) values. Our findings are consistent with late Paleoproterozoic mid-ocean ridge magmatism and primitive arc magmatism succeeded by Mesoproterozoic or Neoproterozoic arc magmatism, a history compatible with previous studies. The Little Dragoon Mountains lie along a boundary between a continental margin assemblage, consisting largely of the Pinal Schist and crosscutting plutons, and an oceanic assemblage of various volcanic rocks with geochemical signatures indicative of both midocean ridge and island arc origins. Some previous workers have postulated that the Pinal Schist in the Little Dragoon Mountains originated as a continental rift or forearc deposit. The schist locally displays original sedimentary characteristics including jumbled fragments of sedimentary strata, mega-blocks of more schistose rock, and melange that contains greenstone bodies and blocks of metamorphosed chert. Our study confirms that much of the Pinal Greenstone, particularly the oldest types (A and B, and possibly D), occurs as lensoidal pods interfoliated with the schist; they are favourably regarded as either olistoliths or tectonically introduced fragments (melange knockers). As such, Pinal Schist in the Little Dragoons is interpreted as a distal part of the Laurentian margin that was affected by collision with, and obduction of, an oceanic terrane consisting mainly of ocean floor, and possibly an island arc. The accretion was part of the Mazatzal orogeny which may also have involved collision with a larger landmass such as eastern Australia and/or Antarctica.
URI
http://hdl.handle.net/10150/676958
Additional Links
https://library.azgs.arizona.edu/
Language
en
Series/Report no.
Arizona Geological Survey Open File Report
Rights
Arizona Geological Survey. All rights reserved.
Collection Information
Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu.
North Bounding Coordinate
35.000
South Bounding Coordinate
31.000
West Bounding Coordinate
-112.000
East Bounding Coordinate
-109.000
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