A nested diapir model for the reversely zoned Turtle Pluton, southeastern California

Abstract

Most zoned plutons described in the geological literature have mafic rims and felsic cores and are referred to as “normally zoned”, whereas relatively few “reversely zoned” intrusions (felsic rims and mafic cores) have been described. That unusual zonation pattern has been variously attributed to in situ processes or to the reordering of an underlying, vertically stratified, magma chamber either by intrusion through an orifice or by emplacement of composite diapirs. The Turtle Pluton is an early Cretaceous, reversely zoned, intrusion that is divided into four facies: a Rim Sequence that is graditionally zoned from bt + ilm + muse monzogranite to hb + bt + mt + sph granodiorite; a Core Facies of more homogeneous hb + bt + mt + sph granodiorite to quartz monzodiorite; between these two facies, a structural discontinuity termed the Schlieren Zone; and an Eastern Facies of monzogranite to granodiorite. Field relationships, distribution of strain, and geochemical and isotopic studies (including a range of initial87Sr/86Sr from 0·7085–0·7065) suggest that the reverse zonation of the Turtle Pluton is the result of sequential emplacement of two diapirs each derived from the same underlying, vertically stratified, magma chamber, and that the Rim Sequence zonation is chiefly the result of mixing of intermediate and felsic magmas from distinct sources accompanied by minor fractional crystallisation.

Polycrystalline amphibole aggregates (clots) in granites as potential I‐type restite: An ion microprobe study of rare‐earth distributions

“…There is no evidence of significant modification of the Strontian granodiorites by diorite magmas as would be shown by linear trends on binary plots (notably Figure 2d). However, there is the possibility that intermediate products of mingling could have been incorporated in the granitic magma in a multistage mingling/mixing process (Allen 1992; Grunder 1995), but would not be disclosed by such linear trends. Amphiboles and biotites rather than pyroxenes are more likely dominant mafic phases in such postulated hybrids, particularly if the granite end‐member had a significant water fugacity.…”

A multidisciplinary approach combining geochemical, gravity and structural data: implications for pluton emplacement and zonation

“…Pluton zonation is generally interpreted as the result of complex geochemical processes that involve wall rock assimilation, preferential accretion of minerals, fractional crystallization or mechanical interactions between magma batches (Ayrton 1988;Fridrich & Mahood 1984;Allen 1992;Stephens 1992;Brandon & Lambert 1994).…”

Granitic batholiths: from pervasive and continuous melting in the lower crust to discontinuous and spaced plutonism in the upper crust

“…Mineralogical zoning resulting in normal (Allen 1992;Stephens 1992) or reversed (Ayuso 1984;Bourne & Danis 1987) zoned plutons proceeds from a similar history. Whether or not magma has time to partly crystallise depends on the time interval between magma inputs.…”