Early arc development recorded in Permian-Triassic plutons of the northern Mojave Desert region, California, USA

M. Robinson Cecil, Mary Ann Ferrer, Nancy R Riggs, Kathleen Marsaglia, Andrew Kylander-Clark, Mihai N. Ducea, Paul Stone

Research output: Contribution to journalArticle

Abstract

Permian-Middle Triassic plutons in the northern Mojave Desert, USA, are emplaced into the cryptic El Paso terrane, which is characterized by a northwest-striking belt of deep marine eugeoclinal strata juxtaposed against Proterozoic basement and its miogeoclinal cover. Fourteen new zircon U-Pb ages from the El Paso Mountains and Lane Mountain region of the Mojave Desert record nearly continuous magmatism occurring between ca. 275 and 240 Ma. These ages, which are taken to record the onset of subduction-related magmatism along the southwestern Laurentian margin, are older than the earliest arc plutons documented in the southern Sierra Nevada region to the north and in the Transverse Ranges to the south. They overlap, however, with Permian arc plutons documented in Sonora, Mexico. Dated plutons are compositionally variable, but can be characterized as intermediate to felsic, calcic to calc-alkalic, and having chemistries consistent with generation in an arc setting. Whole rock Sr-Nd isotopic compositions vary widely from relatively primitive (Sri = 0.7035, initial εNd = +3, initial εHf in zircon = +13) to moderately evolved (Sri = 0.708, initial εNd = -5, initial εHf in zircon = -3). Isotopic signatures differ considerably from partially coeval Triassic suites of the Transverse Ranges and central Mojave, which are more evolved and consistent with emplacement in Proterozoic continental crust of the Mojave province. They also differ considerably from those typical of intermediate plutons generated in intra-oceanic arcs, which are overall much more mantlelike. This suggests that the underpinnings of the El Paso terrane may be at least partly composed of continental crust and that magmas emplaced into the terrane may have been variably contaminated by crustal components. This is supported by the presence of Precambrian and early Paleozoic zircon inheritance recorded in some plutons. In all isotopic systems, values are the most evolved in the oldest plutons (ca. 275-270 Ma), becoming more juvenile in the Middle Triassic. These temporal trends, together with pluton fabrics and new estimates of Permian plate vectors, are interpreted to reflect generation of the earliest arc in a contractional setting that may have driven crustal thickening and a greater involvement of crustal materials in Permian magmas. This result supports a model of forced subduction initiation, which is favored by a change in plate motions along a previously weak margin, and predicts an initial compressive state in the upper plate. The uniformly primitive signatures of Triassic melts are taken to indicate a change to a transtensional upper-plate stress regime that promoted the development of more voluminous, primarily mantle-derived melts. Regional pluton age patterns suggest that arc magmatism initiated in restricted areas of the southwestern Laurentian margin (northern Mojave, Sonora) and then migrated north and south ultimately becoming a continuous arc by Jurassic time.

Original languageEnglish (US)
Pages (from-to)749-765
Number of pages17
JournalBulletin of the Geological Society of America
Volume131
Issue number5-6
DOIs
StatePublished - May 1 2019

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pluton
Permian
Triassic
desert
zircon
magmatism
terrane
continental crust
Proterozoic
subduction
melt
crustal thickening
plate motion
mountain region
Precambrian
emplacement
isotopic composition
Paleozoic
Jurassic
mantle

ASJC Scopus subject areas

  • Geology

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Early arc development recorded in Permian-Triassic plutons of the northern Mojave Desert region, California, USA. / Cecil, M. Robinson; Ferrer, Mary Ann; Riggs, Nancy R; Marsaglia, Kathleen; Kylander-Clark, Andrew; Ducea, Mihai N.; Stone, Paul.

In: Bulletin of the Geological Society of America, Vol. 131, No. 5-6, 01.05.2019, p. 749-765.

Research output: Contribution to journalArticle

Cecil, M. Robinson ; Ferrer, Mary Ann ; Riggs, Nancy R ; Marsaglia, Kathleen ; Kylander-Clark, Andrew ; Ducea, Mihai N. ; Stone, Paul. / Early arc development recorded in Permian-Triassic plutons of the northern Mojave Desert region, California, USA. In: Bulletin of the Geological Society of America. 2019 ; Vol. 131, No. 5-6. pp. 749-765.
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T1 - Early arc development recorded in Permian-Triassic plutons of the northern Mojave Desert region, California, USA

AU - Cecil, M. Robinson

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N2 - Permian-Middle Triassic plutons in the northern Mojave Desert, USA, are emplaced into the cryptic El Paso terrane, which is characterized by a northwest-striking belt of deep marine eugeoclinal strata juxtaposed against Proterozoic basement and its miogeoclinal cover. Fourteen new zircon U-Pb ages from the El Paso Mountains and Lane Mountain region of the Mojave Desert record nearly continuous magmatism occurring between ca. 275 and 240 Ma. These ages, which are taken to record the onset of subduction-related magmatism along the southwestern Laurentian margin, are older than the earliest arc plutons documented in the southern Sierra Nevada region to the north and in the Transverse Ranges to the south. They overlap, however, with Permian arc plutons documented in Sonora, Mexico. Dated plutons are compositionally variable, but can be characterized as intermediate to felsic, calcic to calc-alkalic, and having chemistries consistent with generation in an arc setting. Whole rock Sr-Nd isotopic compositions vary widely from relatively primitive (Sri = 0.7035, initial εNd = +3, initial εHf in zircon = +13) to moderately evolved (Sri = 0.708, initial εNd = -5, initial εHf in zircon = -3). Isotopic signatures differ considerably from partially coeval Triassic suites of the Transverse Ranges and central Mojave, which are more evolved and consistent with emplacement in Proterozoic continental crust of the Mojave province. They also differ considerably from those typical of intermediate plutons generated in intra-oceanic arcs, which are overall much more mantlelike. This suggests that the underpinnings of the El Paso terrane may be at least partly composed of continental crust and that magmas emplaced into the terrane may have been variably contaminated by crustal components. This is supported by the presence of Precambrian and early Paleozoic zircon inheritance recorded in some plutons. In all isotopic systems, values are the most evolved in the oldest plutons (ca. 275-270 Ma), becoming more juvenile in the Middle Triassic. These temporal trends, together with pluton fabrics and new estimates of Permian plate vectors, are interpreted to reflect generation of the earliest arc in a contractional setting that may have driven crustal thickening and a greater involvement of crustal materials in Permian magmas. This result supports a model of forced subduction initiation, which is favored by a change in plate motions along a previously weak margin, and predicts an initial compressive state in the upper plate. The uniformly primitive signatures of Triassic melts are taken to indicate a change to a transtensional upper-plate stress regime that promoted the development of more voluminous, primarily mantle-derived melts. Regional pluton age patterns suggest that arc magmatism initiated in restricted areas of the southwestern Laurentian margin (northern Mojave, Sonora) and then migrated north and south ultimately becoming a continuous arc by Jurassic time.

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