Nested plutons as megageopetal structures: The Merrimac plutons, northern Sierra Nevada, California

Abstract

Contacts within nested plutons are crucial for constraining the relative timing of pluton emplacement and the internal geometry of composite plutons. Exposures in orogenic belts are commonly discontinuous, however, disguising these contacts. In this paper, the Merrimac plutons in the northern Sierra Nevada of California are used as an example of how composition and foliation patterns can allow the definition of unexposed contacts and identify nested plutons.Image analysis techniques were used to determine modal compositions of a total of 52 samples from the Merrimac plutons. The integrated analysis of compositional data and foliations patterns reveals a critical contact within the plutons, and suggests that the Merrimac plutons indicate way-up towards the north-east at the time of emplacement 142 f 3 Ma ago. This paper provides guidelines for recognizing nested plutons in poorly exposed areas and shows that consistent structural and compositional assymmetries within nested plutons can be used as regional top-direction indicators.

Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

“…The potential-field anomalies indicate that the rocks northeast of the gradients are denser and more magnetic relative to rocks southwest of the gradients, which is borne out by density and magnetic susceptibility measurements (Figs. 4 and 5) and is consistent with composition data of Guglielmo (1993a) which show that the northern pluton has somewhat lower potassium feldspar (Fig. 3D) and higher mafic contents.…”

“…The Merrimac plutons are bisected by gravity and aeromagnetic gradients that support the presence of at least two plutons, confirming and refining the original analysis by Guglielmo (1993a). A significant gravity gradient trends N60°W, with higher values to the northeast (Fig.…”

“…This study integrates geophysical, geochronologic, and geomorphic data to examine the role of pre-existing structure in the geometry and composition of the Merrimac plutons. The geophysical data, in particular the aeromagnetic and physical property data, were used to refine the geologic map in Guglielmo (1993a). We used gravity data (Langenheim et al, 2019) to produce isostatic gravity maps (Figs.…”

“…Density and magnetic properties of rock samples are crucial parameters for analyzing and modeling gravity and magnetic data. To augment density measurements of plutons by Hietanen (1973Hietanen ( , 1976 and Larsen and Poldervaart (1961), we measured density and magnetic susceptibility of rock slabs collected for compositional analysis by Guglielmo (1993a) and of additional hand samples of not only the Merrimac plutons (Figs. 4C, 5C; Langenheim et al, 2019) but also of the Bald Rock pluton and country rock, summarized in Table 1.…”

“…Compton (1955) suggested that the compositional zoning of the Bald Rock pluton from trondhjemite in the center to tonalite along the pluton margin was due to contamination. Larsen and Poldervaart (1961) argued for little Guglielmo, 1993a) 134.1 ± 0.6 Ma 140.7 ± 0.7 Ma K-Ar date of 129 and 134 Ma on biotite and hornblende, respectively (Evernden and Kistler, 1970;recalculated in Saucedo, 1992) wall-rock contamination based on the crystal habits of zircon microphenocrysts. Subsequently Sawka (1985) used geochemical and initial strontium isotopic data to rule out wall-rock contamination as a source of compositional zoning, instead arguing for sidewall crystallization and subsequent magma mixing.…”