Aaron S. Yoshinobu

Interpreting magmatic fabric patterns in plutons

Abstract Most plutons have widespread magmatic fabrics, the interpretation of which remains controversial. We propose a method to constrain likely causes of fabric patterns, the application of which indicates the following: (1) preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement; (2) fabrics are poor recorders of total strain and are easily reset, preserving only the last increment of strain during crystallization; (3) in magmatic systems mechanically decoupled from host rocks, patterns may result from strain during internally driven flow, filter pressing or porous flow in relatively static chambers, or by final increments of strain during emplacement; (4) with greater emplacement depths, fabric patterns increasingly reflect strain caused by regional deformation; and (5) given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials, they may provide a relatively direct record of paleostress in orogenic belts.

Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny?

Magmatism, contractional deformation, and extension associated with the exhumation of high-pressure rocks in the Scandinavian Caledonides are commonly attributed to the Silurian-Devonian Scandian orogeny, in which eastward thrusting of allochthonous terranes over Baltica was followed by extensional collapse and exhumation. New fieldwork and U-Pb geochronology coupled with recent pressure-temperature estimates within the highest thrust sequence of the Caledonian orogen indicate that an earlier phase of westdirected contractional deformation was punctuated by migmatite-producing events and voluminous magmatism ca. 477‐466 Ma and ca. 447 Ma, followed by exhumation in the Late Ordovician. Al-in-hornblende and GASP thermobarometry indicate that emplacement of a suite of 448‐445 Ma plutons caused partial migmatization at pressures of 700‐ 800 MPa. Subsequent isothermal exhumation to pressures of 400 MPa occurred while the host rocks were still partially molten. Rates of exhumation may have ranged from 2 to 11 mm·yr 21 or greater. These data provide evidence for a previously unrecognized phase of exhumation in the Caledonides and for aerially extensive west-vergent deformation. Deformation and magmatism associated with these events may be related to Taconic-age orogenesis near Laurentia, where the highest nappe sequences of the Scandinavian Caledonides probably resided during early Paleozoic time.

Modeling the thermal evolution of fault-controlled magma emplacement models: implications for the solidification of granitoid plutons

A two-dimensional finite diAerence model is used to simulate the conductive thermal regime attend- ing construction and maintenance of a continental magma chamber by intrusion of granite dikes into grano- diorite host rocks displaced at various spreading rates. Final intrusion shapes include tabular, square, and vertical rectangular bodies emplaced in the shallow crust (5-15 km) and tabular bodies emplaced in the middle crust (15-20 km) fed by dikes with widths of between 20 and 100 m. The formation of a steady-state chamber is defined as the point at which the ambient temperatures surpass the intrusion solidus forestalling the solidifi- cation of subsequently intruded material. For spreading rates <10 mm year ˇ1 , construction of a steady-state magma chamber in the shallow crust took 260 ka (rectangular), 360 ka (square), and 1 Ma (tabular), whereas in the mid crust a steady state was reached in less than 30 ka (tabular). At faster spreading rates (25 and 50 mm year ˇ1 ) ambient temperatures pass the solidus isotherm forming a steady-state reservoir within 55 ka, depending on intrusion depth and size. For 10-25 mm year ˇ1 spreading rates, sheeted dikes make up from 10 to 100% of the intrusion. The thermal modeling supports the following conclusions: (a) episodic magma emplacement into a fault-con- trolled setting is a thermally viable means of constructing a steady-state chamber at moderate to fast spreading rates only if the duration of faulting and intrusion are long enough to elevate ambient temperatures above the intrusion solidus, (b) isotherms will migrate outward during successive intrusion before converging back on the center of the intrusion after chamber construction, (c) the margins of most intrusions formed by this scen- ario should contain sheeted dikes, (d) the solidus isotherm, and thus the solidification front that it tracks, will become progressively curviplanar during the construction of the magma chamber and will not represent the in- itial shape of the intrusions (i.e. sheets), (e) the steady-state chamber will be smaller than the total intrusion dimensions, and (f) magmatic fabrics will form diachronously and not always parallel to sheet margins as they track the migrating solidification front. Because it is unlikely that most large intrusions formed instan- taneously, the eAects of continued addition of heat on the migration of solidification fronts may have signifi- cant implications for magmatic processes in many emplacement scenarios. # 1998 Elsevier Science Ltd. All rights reserved

A view from the roof: magmatic stoping in the shallow crust, Chita pluton, Argentina

Nearly 2 km of vertical relief and exposure of continuous pluton roof-wall corners around the Chita pluton, northwest Argentina, provides constraints on the 3-D host rock displacement field attending magma emplacement in the shallow crust. The pluton is rectangular in crosssectional view and consists of weakly to non-deformed granite to granodiorite. Structures in surrounding metasedimentary host rocks are truncated at a knife-sharp contact and are only weakly deflected from their regional orientations. Final emplacement occurred by stoping together with minor doming of the earth’s surface. Structural restoration of a cross-section indicates that host-rocks above the pluton may have been domed upwards during emplacement by as much as a few hundred meters, accounting for up to 20% of the exposed pluton volume. The remaining 80% of the space for pluton emplacement was made by downward removal of host-rock from the present level of exposure. Although mechanisms such as floor-subsidence are permissive, late stoping has destroyed evidence for the early emplacement history. However, geometrical constraints provided by exposure of continuous wall-to-roof contacts indicate that early emplacement mechanisms at this level in the crust and immediately below were also dominated by the downward displacement of host rocks through a conduit that now is represented by the solidified pluton. q 2002 Elsevier Science Ltd. All rights reserved.

Midcrustal emplacement of the Sausfjellet pluton, central Norway: Ductile flow, stoping, and in situ assimilation

Midcrustal (25–30 km) emplacement of the dioritic Sausfjellet pluton into rocks of the Helgeland Nappe Complex, central Norway, occurred in two stages. Stage 1 consists of two-pyroxene hornblende gabbro and diorite. Stage 2 is asymmetrically zoned, with a modally layered central zone of diorite and anorthosite and a western/annular zone of quartz-bearing monzodioritic rocks. Igneous layering was locally attenuated, folded, and boudinaged in the hyper-solidus state. The magmatic foliation trajectory pattern in the pluton defi nes a shallowly southwest-plunging synform that crosscuts compositional zones. Mineral lineations plunge shallowly to moderately to the southwest. The pluton intruded a major lithologic boundary within the nappe; migmatitic pelitic gneisses are the main host rocks to the western part of the pluton, whereas the eastern and central parts are hosted predominantly by metacarbonate rocks. Calc-silicate, marble, quartzo-feldspathic, and dioritic xenoliths (up to 200 m in length) are present throughout the pluton; they are most common in Stage 1 and the central zone of Stage 2. Metapelitic xenoliths are conspicuously absent. Ductile fl ow during emplacement produced an ~1-kmwide structural aureole in which host-rock structures were defl ected into subparallelism with the steeply inward-dipping margin of the pluton. Tight antiforms developed along the northeastern, southeastern, and southwestern margins. Amphibolite-grade shear zones in the host rocks preserve plutonside-up kinematic indicators. In addition to the abundance of dioritic, calc-silicate, and quartzo-feldspathic gneiss xenoliths and geochemical evidence for assimilation in the western/annular zone, regional discordance of the pluton–host-rock contacts indicates that stoping was also an important emplacement process at midcrustal depths. Following magma emplacement, foundering of the central portion of the chamber combined with possible ca. 445 Ma regional contraction produced the map-scale synform defi ned by magmatic foliations and igneous layers. This study demonstrates that stoping and assimilation may occur simultaneously with host-rock ductile fl ow during magma chamber evolution at midcrustal levels and offers an explanation of why xenolith preservation may be compositionally dependent.

Batch-wise assembly and zoning of a tilted calc-alkaline batholith: Field relations, timing, and compositional variation

The Wooley Creek batholith is a tilted, zoned, calc-alkaline plutonic complex in the Klamath Mountains, northern California, USA. It consists of three main compositional-temporal zones. The lower zone consists of gabbro through tonalite. Textural heterogeneities on the scale of tens to hundreds of meters combined with bulk-rock data suggest that it was assembled from numerous magma batches that did not interact extensively with one another despite the lack of sharp contacts and identical ages of two lower zone samples (U-Pb [zircon] chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages of 158.99 ± 0.17 and 159.22 ± 0.10 Ma). The upper zone is slightly younger, with 3 samples yielding ages from 158.25 ± 0.46 to 158.21 ± 0.17 Ma, and is upwardly zoned from tonalite to granite. This zoning can be explained by crystal-liquid separation and is related to upward increases in the proportions of interstitial K-feldspar and quartz. Porphyritic dacitic to rhyodacitic roof dikes have compositions coincident with evolved samples of the upper zone. These data indicate that the upper zone was an eruptible mush that crystallized from a nearly homogeneous parental magma that evolved primarily by upward percolation of interstitial melt. The central zone is a recharge area with variably disrupted synplutonic dikes and swarms of mafic enclaves. Central zone ages (159.01 ± 0.20 to 158.30 ± 0.16 Ma) are similar to both lower and upper zones crystallization ages. In the main part of the Wooley Creek batholith, age data constrain magmatism to a short period of time (

Is stoping a volumetrically significant pluton emplacement process?: Discussion

[Glazner and Bartley (2006)][1] cite and interpret five lines of evidence to indicate that magmatic stoping is unlikely to be a volumetrically significant process during pluton emplacement. Because of the iconoclastic tone of the paper, it is useful to examine each line of evidence cited by Glazner

Measuring host rock volume changes during magma emplacement

We use a technique to evaluate volume strains in contact aureoles during magma emplacement based on geochemical mass balance calculations. Dynamothermal contact metamorphism adjacent to the Emigrant Gap composite pluton, Sierra Nevada, California, produced an 01 km wide thermal/structural aureole consisting of an outer andalusite2cordierite zone and an inner potassium feldspar2sillimanite zone. Mass balance calculations using Al as an immobile reference frame element in metapelites inside and outside of the aureole indicate that the percentage change in total rock mass between chlorite-grade rocks outside of the thermal eAects of the pluton and the aureole isˇ11.1%21.4%. Mass balance calculations indicate that Si was the major rock forming element depleted (0ˇ16%) from the aureole during contact metamorphism. Mass balance calculations and density measurements yield volume strains associated with mass transfer during contact metamorphism of ˇ12.4%. These data, in conjunction with field relationships, are interpreted to suggest that (a) element mobility during contact metamorphism is not restricted to volatiles, and (b) volume losses within the contact aureole occur during magma emplacement and may contribute to the ‘space-making’ process during magma emplacement if intrusion spans the period necessary to engage hydrothermal circulation in the host rocks. # 1998 Elsevier Science Ltd. All rights reserved.

Determining relative magma and host rock xenolith rheology during magmatic fabric formation in plutons: Examples from the middle and upper crust

Field observations, structural analysis, and analytical calculations are utilized to evaluate the strength of intermediate magmas during crystallization in a regional strain field. Two plutons are examined, the subvolcanic 98 Ma old Jackass Lakes pluton, central Sierra Nevada, California, and the voluminous middle crustal 442 Ma old Andalshatten pluton, central Norway. The Andalshatten example contains millimeter- to kilometer-scale xenoliths that display evidence for synmagmatic deformation, including fold reactivation and boudinage, after being isolated in the magma. Fabrics within the pluton adjacent to the xenoliths are usually magmatic, with only local, discontinuous zones of crystal-plastic deformation <1 m from the xenolith contact. Examination of particularly well exposed mafic metavolcanic xenoliths in the Jackass Lakes pluton indicates that all were strained prior to incorporation and then separated from the remaining host rock by brittle cracking. Once isolated from the host rocks, some of these xenoliths were intruded by veins fed by the in situ draining of melt and magma from the surrounding crystal mush zone. The xenoliths continued to deform ductilely at presumably fast strain rates. Axial-planar magmatic foliations within folded granodioritic dikes within xenoliths are parallel to magmatic foliations throughout the Jackass Lakes pluton and metamorphic foliations within the host rocks, indicating that the xenolith deformation occurred within the regional 98 Ma old strain field that affected the pluton. The behavior of these xenoliths suggests that late in the crystallization history, magmas in both middle crustal and subvolcanic settings behaved as a high-strength crystal-melt mush capable of transmitting deviatoric stresses, which drove both elastic and plastic deformation in the enclosed xenoliths. Simultaneously, intercrystalline melt, and in some cases magma, was drained from the host intrusions into the xenoliths. Rheological modeling based on geochemical data yields an effective viscosity of a crystal-free melt of ~104 Pa s and increased to ~107 Pa s as cooling proceeded to 758 °C and crystal content approached 40% for the Jackass Lakes pluton. Such viscosities are too low to impart or transmit deformation into the xenoliths. The preservation of xenoliths in both plutons is compatible with higher crystallinities and/or magma yield strengths as an explanation to arrest the xenoliths in their final position and allow deformation. Estimated effective viscosities considering magma yield strength and measured density variables (melt and solid) are ~1013 Pa s.

Use of trace element abundances in augite and hornblende to determine the size, connectivity, timing, and evolution of magma batches in a tilted batholith

The tilted Wooley Creek batholith (Klamath Mountains, California, USA) consists of three main zones. Field and textural relationships in the older lower zone suggest batchwise emplacement. However, compositions of augite from individual samples plot along individually distinct fractionation trends, confi rming emplacement as magma batches that did not interact extensively. The younger upper zone is upwardly zoned from tonalite to granite. Major and trace element compositions of hornblende show similar variations from sample to sample, indicating growth from a single magma batch that was homogenized by convection and then evolved via upward percolation of interstitial melt. Highly porphyritic dacitic roof dikes, the hornblende compositions of which match those of upper zone rocks, demonstrate that the upper zone mush was eruptible. The central zone contains rocks of both lower and upper zone age, although in most samples hornblende compositions match those of the upper zone. The zone is rich in synplutonic dikes and mafi c magmatic enclaves. These features indicate that the central zone was a broad transition zone between upper and lower parts of the batholith and preserves part of the feeder system to the upper zone. Homogenization of the upper zone was probably triggered by the arrival of mafi c magma in the central zone. Continued emplacement of mafi c magmas may have provided heat that permitted differentiation of the upper zone magma by upward melt percolation. This study illustrates the potential for use of trace element compositions and variation in rock-forming minerals to identify individual magma batches, assess interactions between them, and characterize magmatic processes.