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Determining Relative Magma and Host Rock Xenolith Rheology During Magmatic Fabric Formation in Plutons: Examples from the Middle and Upper Crust

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Determining Relative Magma and Host Rock Xenolith Rheology During Magmatic Fabric Formation in Plutons: Examples from the Middle and Upper Crust Determining relative magma and host rock xenolith rheology during magmatic fabric formation in plutons: Examples from the middle and upper crust Aaron S. Yoshinobu* Jeannette M. Wolak*† Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA Scott R. Paterson* Geoffrey S. Pignotta*§ Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA Heather S. Anderson* Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA ABSTRACT tilely at presumably fast strain rates. Axial- to arrest the xenoliths in their fi nal position planar magmatic foliations within folded and allow deformation. Estimated effective Field observations, structural analysis, and granodioritic dikes within xenoliths are par- viscosities considering magma yield strength analytical calculations are utilized to evaluate allel to magmatic foliations throughout the and measured density variables (melt and the strength of intermediate magmas during Jackass Lakes pluton and metamorphic foli- solid) are ~1013 Pa s. crystallization in a regional strain fi eld. Two ations within the host rocks, indicating that plutons are examined, the subvolcanic 98 Ma the xenolith deformation occurred within the INTRODUCTION old Jackass Lakes pluton, central Sierra regional 98 Ma old strain fi eld that affected Nevada, California, and the voluminous mid- the pluton. The strength of magmas as they crystallize dle crustal 442 Ma old Andalshatten pluton, The behavior of these xenoliths suggests remains poorly constrained in natural environ- central Norway. The Andalshatten example that late in the crystallization history, mag- ments. However, the rheological transitions that contains millimeter- to kilometer-scale xeno- mas in both middle crustal and subvolcanic occur during crystallization (and melting) must liths that display evidence for synmagmatic settings behaved as a high-strength crystal- play a pivotal role in the mechanical evolution deformation, including fold reactivation melt mush capable of transmitting deviatoric of the lithosphere, the segregation and migra- and boudinage, after being isolated in the stresses, which drove both elastic and plastic tion of magma, and the eventual solidifi cation magma. Fabrics within the pluton adjacent deformation in the enclosed xenoliths. Simul- and thus emplacement of igneous bodies. In this to the xenoliths are usually magmatic, with taneously, intercrystalline melt, and in some article we explore how magma-xenolith rela- only local, discontinuous zones of crystal- cases magma, was drained from the host tions in plutons may provide information on the plastic deformation <1 m from the xenolith intrusions into the xenoliths. Rheological viscosity of partially molten systems and the contact. Examination of particularly well modeling based on geochemical data yields strength of magmas as they crystallize. Many exposed mafi c metavolcanic xenoliths in the an effective viscosity of a crystal-free melt of experimental and theoretical investigations have Jackass Lakes pluton indicates that all were ~104 Pa s and increased to ~107 Pa s as cool- demonstrated the complexity and diffi culty in strained prior to incorporation and then ing proceeded to 758 °C and crystal content quantifying the rheology of magmas because of separated from the remaining host rock by approached 40% for the Jackass Lakes plu- their multiphase nature across a range of tem- brittle cracking. Once isolated from the host ton. Such viscosities are too low to impart or peratures, pressures, ambient deviatoric stresses, rocks, some of these xenoliths were intruded transmit deformation into the xenoliths. The and fl uid compositions and/or concentrations by veins fed by the in situ draining of melt and preservation of xenoliths in both plutons is (e.g., Arzi, 1978; Van der Molen and Paterson, magma from the surrounding crystal mush compatible with higher crystallinities and/ 1979; McBirney, 1993; Lejeune and Richet, zone. The xenoliths continued to deform duc- or magma yield strengths as an explanation 1995; Rutter and Neumann, 1995; Barboza *Emails: Yoshinobu: [email protected]; Wolak: [email protected]; Paterson: [email protected]; Pignotta: [email protected]; Anderson: [email protected]. †Present address: Department of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA. §Present address: Department of Geology, University of Wisconsin-Eau Claire, Eau Claire, Wisconsin 54702, USA. Geosphere; June 2009; v. 5; no. 3; p. 270–285; doi: 10.1130/GES00191.1; 12 fi gures. 270 For permission to copy, contact [email protected] © 2009 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/3/270/3338380/i1553-040X-5-3-270.pdf by guest on 30 September 2021 Relative magma and host rock xenolith rheology during fabric formation and Bergantz, 1998; Renner et al., 2000; Rosen- ination of xenoliths in both plutons indicates includes diorite, quartz diorite, biotite ± horn- berg, 2001; Petford, 2003). Our approach is to that they were all pervasively deformed during blende granodiorite, granite, leucogranite, and utilize fi eld relations, structural analysis, and magmatic fabric formation. While this ductile additional hybrid phases (McNulty et al., 1996; geochemistry to place constraints on the rheol- deformation was ongoing, the xenoliths were Coyne et al., 2004). Generally these various ogy of magmas during xenolith incorporation brittlely cracked, sometimes injected by melts phases form north-northwest–trending, steeply and deformation in shallow crustal and middle and magmas from the surrounding magma, and dipping sheet-like bodies ranging in width from crustal environments. Our results may bear in some cases these veins were boudinaged and/ meters to kilometers with variable contacts on the effi cacy of magma emplacement pro- or folded at relatively fast strain rates. ranging from gradational to locally sharp. How- cesses such as diking and stoping, as well as The above observations are interpreted to ever, the central and western portions of the plu- provide constraints on magma viscosity at high indicate that late in the crystallization history ton contain distinct compositional units several crystal fractions. of both plutons, the magma behaved as a high- kilometers in width with quite variable shapes, In this paper we use the term xenolith to strength crystal-melt mush capable of trans- although generally elongate in the north- describe any body of rock that is foreign to and mitting deviatoric stresses, which drove both west direction. Mafi c microgranitoid enclave entirely surrounded by the host igneous rock elastic and plastic deformation in the xenoliths, swarms of dioritic composition as wide as 50 m in contact with the xenolith. This may include while intercrystalline melt drained from the host occur in the more felsic units and are often stoped blocks from the roof, walls, or fl oor of the magma into the xenoliths. Furthermore, com- northwest trending. Textures vary considerably, magma chamber, cognate xenoliths (autoliths) parison of structures in the host pluton and in from aphanitic to coarse grained, and from equi- derived from earlier crystallized portions of the the xenoliths indicates that the strength of the granular to porphyritic. magma system, and screens (kilometric-scale crystal-melt mush during magmatic fabric for- Numerous metavolcanic and metasedimen- xenoliths, sometimes referred to as pendants), mation was equal to or greater than that of the tary xenoliths occur throughout the Jackass or raft trains of xenoliths (e.g., Pitcher, 1970). metavolcanic xenoliths in the Jackass Lakes Lakes pluton and vary from kilometer-scale We differentiate xenoliths from mafi c magmatic pluton. The magmatic foliations and lineations screens to millimeter-scale xenoliths (Figs. 1 enclaves or microgranitoid enclaves (e.g., Didier in both plutons are interpreted to refl ect regional and 2). The largest screens resemble lithologies and Barbarin, 1991). Xenocrysts are defi ned as strain. Thus, both plutons preserve evidence for exposed in the Minarets caldera sequence to mineral fragments that are encapsulated in the the orientation of the regional strain fi eld during the east (Fig. 1) and include both metavolcanic host igneous rock and have no direct chemical magma emplacement. and metasedimentary rocks. Some pendants are relationship with the host magma. The study of also intruded by a porphyritic leucogranite with such xenoliths may provide information about XENOLITHS IN THE JACKASS LAKES miarolitic cavities called the Post Peak phase, magma compositional changes due to assimila- PLUTON, CENTRAL SIERRA NEVADA, which probably is a slightly earlier, subvolcanic tion (e.g., Barnes et al., 2004), the paleohori- CALIFORNIA leucogranitic phase of the Jackass Lakes pluton zontal at the time of fi nal chamber construction, (Peck, 1980). Individual xenoliths and/or raft timing of formation of structures in plutons, The Jackass Lakes pluton (Fig. 1) in map view trains of xenoliths are particularly common near kinematics of magma fl ow, and the rheology is an ~13 × 17 km rectangular body that intruded the large pendants, but occur throughout the plu- of magmas at the time of xenolith capture (e.g., slightly older metavolcanic and plutonic pen- ton with variable sizes (meters to several hun- Paterson and Miller, 1998). Xenoliths also may dants and screens (Peck, 1980; McNulty et dred meters) and rock types (metavolcanic, leu-
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