Scott R. Paterson

A review of criteria for the identification of magmatic and tectonic foliations in granitoids

Abstract Foliations in granitoids can form by magmatic flow, ‘submagmatic flow’, high-temperature solid-state deformation and moderate- to low-temperature solid-state deformation. A review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent, diapiric emplacement and expansion, emplacement during regional deformation, or regional deformation post-dating emplacement. However, a magmatic origin is favoured for foliations defined by the alignment of igneous, commonly euhedral minerals, particularly where the foliation is parallel to internal or external pluton contacts. Foliations formed during expansion or ‘ballooning’ of diapirs may be strictly magmatic in origin, although some studies suggest that solid-state deformation also may occur. If so, we would hope to find evidence of deformation of crystal-melt systems, and that the solid-state deformation occurred at high temperatures. The inference of syntectonic foliations is most convincing where magmatic and high-temperature solid-state foliations are subparallel, these foliations are continuous with regionally developed foliations in the wall rocks, synkinematic porphyroblasts are present in the wallrocks, and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage. A strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals, no alignment of igneous minerals occurs, the foliation is locally at high angles to pluton-wallrock contacts, and the foliation is continuous with a regionally developed cleavage.

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.

Re-examining pluton emplacement processes

Abstract Previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement. In fact, the only means of ‘making space’ during emplacement of mantle-derived magmas in the crust are (1) lowering the Moho or (2) outward displacement of the Earths surface. Other ‘pluton emplacement mechanisms’ are material transfer processes (MTPs) that do not increase the volume of the crust. One frequently cited MTP for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall-rocks around the plutons. The expected structures and total strains near such plutons are dependent on the three-dimensional pluton shape, the width of the deforming aureole, whether the pluton is a piercing or non-piercing diapir, and, in the latter case, the number of body radii the pluton travels. We have re-examined a few such plutons and calculated the amount of material transfer caused by ductile flow. In no case is more than 40%, and in most cases only 15–35%, of the required material transfer accommodated by wall-rock flow. The contact aureoles around these plutons are too narrow and/or strains too low, indicating that the plutons are predominantly discordant bodies with narrow (0.1–0.4 pluton radii) concordant aureoles. A comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains. We suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic. Instead, we argue that the ascent and emplacement of magmas require multiple near-field and far-field MTPs. These MTPs will show gradients with depth, distance from the magma, and with time. Rates of near-field MTPs must be rapid, whereas far-field rates are probably nearer to long-term average rates of orogenic processes.

Bursting the bubble of ballooning plutons: A return to nested diapirs emplaced by multiple processes

A popular model for the emplacement of roughly spherical plutons is that of “ballooning” or in situ inflation of a magma chamber. In a common version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent, while the hotter “tail” of magma continues to rise and expand the already crystallized outer margin. This expansion forms a concentric, gneissic to mylonitic foliation and flattening-type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the surrounding country rock to form a dynamothermal aureole that postdates regional structures. Our reexamination of three supposedly ballooned plutons (Ardara, Ireland; Cannibal Creek, Australia; Papoose Flat, California) and evaluation of published descriptions of many others indicate that this model is largely incorrect. Deflections of country-rock structures, strains, and porphyroblast-matrix relationships indicate that only minor to moderate expansion (usually 30% or less) occurred during emplacement, that other emplacement mechanisms must have occurred, and that regional deformation continued during and after emplacement. Internal structures indicate that when magma chamber expansion did occur, it did so by flow of magma, that magmatic foliations and lineations formed late in the magma chamber evolution, that enclave shapes are neither good strain markers nor indicators of the magnitude of expansion, and that only minor internal subsolidus deformation results from emplacement. This study indicates that many plutons previously interpreted as post-tectonic ballooning plutons are better viewed as syntectonic, nested diapirs emplaced by a variety of country-rock material-transfer processes. This nested diapir model implies that magma ascent may occur by rise of large magma batches (instead of transport in dikes followed by ballooning), that magma chamber dynamics differ from that in the ballooning model, and that normally zoned plutons may form by intrusion of several pulses of magma rather than by in situ crystal fractionation from a single parent melt.

Rates of processes in magmatic arcs: implications for the timing and nature of pluton emplacement and wall rock deformation

Abstract The construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions. It is critical, therefore, to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton-wall rock systems. Our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following: (1) crystal growth rates in magma of 10 −4 cm year −1 ; (2) growth rates of metamorphic porphyroblasts between 10 −5 and 10 −2 cm year −1 ; (3) long-term magma supply rates of 10 −1 km 3 year −1 and short-term rates of up to 350 km 3 year −1 ; (4) diapiric ascent rates for mafic plutons of 1–3 m year −1 ; (5) cooling of plutons to ambient wall rock temperatures in 10 5 –10 6 years; (6) final crystallization of plutons in a small fraction of the time needed for complete cooling; (7) fault displacements of 3 cm year −1 ; (8) development of cleavages in fault zones in less than 10 6 years at strain rates of 10 −13 s −1 or higher; and (9) the development of regional cleavages in 10 6 years at strain rates of 10 −14 s −1 These rates indicate that processes operating in magmatic arcs are relatively rapid: pluton emplacement, cleavage development, etc., occur over time spans of tens of thousands to no more than a few million years. However, the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed. At deeper levels, or in zones undergoing faster strain rates, the time spans of the various processes approach one another. Thus plutons, with otherwise similar characteristics, emplaced in regions undergoing fast strain rates, or at deeper crustal levels, may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates. Comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered. Thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration. Finally, we argue that the structural and other characteristics of pluton-wall rock systems will depend on rates of various processes involved and that these rates at the very least influence, and sometimes invalidate, the timing criteria previously published by us and others.

Using pluton ages to date regional deformations: Problems with commonly used criteria

Studies of pretectonic plutons in the Foothills terrane, central Sierra Nevada, California, along with a review of studies on syntectonic and post-tectonic plutons, indicate that no single criterion can establish the relative timing of pluton emplacement and regional deformation. Criteria most often used are whether a pluton is deformed, cuts regional structures, or is associated with porphyroblasts that postdate regional structures. However, all three types of plutons may (1) appear deformed or nondeformed, (2) have contacts that appear to cut structures in the wall rocks, and (3) have porphyroblasts in their aureoles that display varied timing relations. The ambiguity of these criteria emphasize the need for careful studies of the structures present throughout the pluton and surrounding wall rock, with particular attention paid to regions where the pluton contact is at high angles to regional structures. Porphyroblast-cleavage relations are an important tool in this regard, but also must be used with caution.

Analysis and interpretation of composite foliations in areas of progressive deformation

Abstract In areas of progressive deformation, where successive structures develop during a relatively continuous deformation within a geologically short time period, traditional chronological notations of structural elements (e.g. S 1 , S 2 , etc.) can give erroneous impressions of how large rock masses evolve in time and space. We demonstrate from field examples that successive structures can develop which: (a) are comparable in morphology and orientation but of different ages; (b) are different in morphology but of comparable age; and (c) show rapid morphological changes over short distances. Under such conditions, correct identification of the relative age of structures is often difficult to impossible. We consider the concepts of a composite foliation , and of Transposition Cycles as vehicles to objectively evaluate the significance of different sets of structures in the evolution of larger rock masses. We suggest that: (1) structural elements be labeled using morphological notation, adding numerical subscripts only when independent evidence is available; (2) geologists more fully acknowledge and integrate the concept of deformation partitioning into their models; and (3) when analyzing areas of multiple deformation, more emphasis is placed on the relationship between domains of differing complexity. Integration of these three perspectives in the analysis will lead to a more realistic basis upon which to model the structural evolution of large rock masses.

The transition from magmatic to high-temperature solid-state deformation: implications from the Mount Stuart batholith, Washington

Criteria for syntectonic emplacement of plutons are commonly ambiguous. The strongest single criterion is probably the preservation of a continuous transition from submagmatic to high-temperature solidstate deformation. This transition is not commonly documented. An exception is the Mount Stuart batholith, which displays a variety of syntectonic structures within its sheared northeastern margin, including the following: (1) S-C fabrics that range from submagmatic to high-temperature solid-state;(2) solid-state foliation and lineation defined by amphibolite-facies assemblages that overprint and are parallel to magmatic foliation and lineation; (3) folds of magmatic and solid-state foliation that have hinge lines parallel to magmatic and solid-state lineation and to equivalent structures in wall rocks; (4) dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite; (5) pegmatite dikes and mineralized joints that are sub-perpendicular to magmatic and solid-state lineation; and (6) submagmatic and high-temperature solidstate ductile shear zones. We suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of postemplacement deformation. Syntectonic features are most likely preserved in plutons cooled at slow to moderate rates, or in plutons deformed at high strain rates during emplacement. The optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid-crust, such as the Mount Stuart batholith, and in intrusions at deeper levels that were rapidly exhumed and/or intruded during the waning stages of regional deformation.

Magma addition and flux calculations of incrementally constructed magma chambers in continental margin arcs: Combined field, geochronologic, and thermal modeling studies

Incrementally constructed magma systems have been recognized from studies of the resulting plutons for more than three decades. However, magma addition rates, fluxes, growth durations, sizes of increments, and sizes and durations of the resulting magma chambers have been difficult to ascertain, emphasizing the need for a better understanding of how magmatic systems evolve. Our results from studies of plutons and arc sections in the North American Cordillera indicate that a large range exists in all of these values. Although arc sections and individual plutons clearly have dramatic temporal changes in volumetric magma additions, true volumetric flux calculations are particularly difficult to determine. Thus, although subduction beneath arcs may have active durations of hundreds of millions of years, volumetrically most magmatism is emplaced during magmatic flare-ups of ∼10–30 m.y. duration. Individual plutons and batholiths in these arcs can grow in

Magmatic lobes as "snapshots" of magma chamber growth and evolution in large, composite batholiths: An example from the Tuolumne intrusion, Sierra Nevada, California

Precise chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages in combination with detailed field mapping, 40 Ar/ 39 Ar thermochronology, and finite difference thermal modeling in the magmatic lobes of the Tuolumne batholith characterize these 10–60 km 2 bodies as shorter-lived, simpler magmatic systems that represent increments of batholith growth. Lobes provide shorter-term records of internal and external processes that are potentially obliterated in the main body of long-lived, composite batholiths. Zircon ages complemented by thermal modeling indicate that lobe-sized magma chambers were present between ∼0.2 and 1 m.y., representing only a small fraction of the total duration of melt presence in the main body. During these shorter intervals, a concentric pattern of normal compositional zoning formed during inward crystallization and widespread zircon recycling in the lobes. Lobes largely evolved as individual magma bodies that did not interact significantly with the main, more complex magma chamber(s). Antecrystic zircons and the range of autocrysts, used to track the extent of interconnected melt, record only a limited range of ages and have contrasting zircon populations to those found in the same units in the main batholith. We consider lobes to either be single batches formed during continuous magma flow or multiple, quickly coalescing pulses that in either case formed separate magma chambers that failed to amalgamate with other compositionally distinct pulses such as those occurring in the central batholith. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary, but that the locus of magmatism shifted both inward and northwestward.