Counterclockwise P-T-t Paths from Amphibolites, Franciscan Complex, California: Relics from the Early Stages of Subduction Zone Metamorphism

Overprinting mineral relationships in amphibolites from the Franciscan Complex, California suggest metamorphic evolution from high-temperature amphibolite facies to the blueschist facies with increasing P/T ratio, a counterclockwise P-T-t path. Early formed pargasitic amphiboles are both crosscut an...

Ausführliche Beschreibung

Gespeichert in:
Bibliographische Detailangaben
Veröffentlicht in:The Journal of geology 1990-09, Vol.98 (5), p.657-680
1. Verfasser: Wakabayashi, John
Format: Artikel
Sprache:eng
Schlagworte:
Online-Zugang:Volltext
Tags: Tag hinzufügen
Keine Tags, Fügen Sie den ersten Tag hinzu!
Beschreibung
Zusammenfassung:Overprinting mineral relationships in amphibolites from the Franciscan Complex, California suggest metamorphic evolution from high-temperature amphibolite facies to the blueschist facies with increasing P/T ratio, a counterclockwise P-T-t path. Early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are, in turn, cut or rimmed by sodic amphiboles. Some amphibolites are overprinted successively by eclogite and blueschist facies assemblages. Clinopyroxenes are zoned with increasing jadeite component from core to rim. Multiple generations of phengites are present in the same sample and are more Si-rich with each successive generation of growth. Garnets in eclogite-overprinted amphibolites are zoned with an initial MgO increase away from the core to a maximum MgO zone followed by MgO decrease toward the rim. Garnets in other amphibolites show continually increasing MgO from core to rim. Several samples, representative of the variety of Franciscan amphibolites, are described in detail. The best-constrained sample exhibits a calculated P-T trajectory from 626-664°C at 9.2-10 kb for early amphibolite metamorphism to 496-537°C at 11.2-11.8 kb eclogite metamorphism to 300-350°C at >6.5-7 kb for final blueschist overprint. Overprinting mineral assemblages in other samples, as well as most Franciscan amphibolites described by other workers, are qualitatively, and in some cases quantitatively, suggestive of the same type of P-T trajectory. Geochronologic, textural, and petrologic data indicate that the overprinted amphibolites are probably the product of a single metamorphic event rather than a consequence of separate metamorphic episodes, but additional age dating is desirable for better confirmation. Amphibolite to blueschist facies metamorphic evolution may have taken place within 5 Ma. The following model is presented for the evolution of Franciscan amphibolites: (1) The amphibolites were metamorphosed as a dynamothermal aureole underneath the hanging wall of a subduction zone at the inception of subduction. (2) The amphibolites were then accreted to the upper plate. (3) Subsequent subduction and underplating of cold material insulated the hanging wall and allowed the upper plate and amphibolite to cool at depth, with the development of metamorphic assemblages of increasing P/T ratio. (4) Subduction beneath the accreted amphibolite may have broken up the amphibolite sheet, dragging blocks deeper down the subduction zone, leadi
ISSN:0022-1376
1537-5269
DOI:10.1086/629432