Igneous Rock Associations 7. Arc Magmatism I: Relationship Between Subduction and Magma Genesis J

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Geoscience Canada

Igneous Rock Associations 7. Arc Magmatism I: Relationship Between Subduction and Magma Genesis J. Brendan Murphy

Volume 33, Number 4, December 2006 Article abstract Dehydration of subducted oceanic lithosphere releases fluid into the overlying URI: https://id.erudit.org/iderudit/geocan33_4ser01 mantle wedge and initiates a chain of events culminating in the generation of magma that rises to form volcanic arcs. Arc magmas are distinct from magmas See table of contents in other settings because of the different geothermal regimes at destructive plate margins, the presence of fluids/volatiles derived from dehydration of the subducted slab and the generally compressive tectonic regime that inhibits the Publisher(s) ascent of magma, thereby promoting extensive interaction with the adjacent wall rocks. As a result, most arc magmas solidify as intrusive bodies, ranging The Geological Association of Canada from sills and dykes to large plutonic complexes. Several mechanisms facilitate the rise of arc magmas. Diapirs rising from a larger pool of buoyant magma are ISSN important in the ductile lower crust. The rapid rise and the expansion of magma results in the propagation of fractures, that facilitate stoping and 0315-0941 (print) assimilation in the brittle upper crust. Fracture zones are repeatedly exploited, 1911-4850 (digital) and the net result may be formation of a composite batholith. Water plays an important role in all stages of arc-magma evolution. Water Explore this journal lowers the temperatures that are required for the partial melting in the mantle wedge, which produces mafic magma, and in the crust, produces felsic magma. Arc magmatism is intimately related to metamorphism, although this Cite this article relationship is complicated largely because maximum pressures and temperatures are attained at different times. Arcs under compression undergo Murphy, J. B. (2006). Igneous Rock Associations 7. Arc Magmatism I:: rapid thickening, followed by erosional or tectonic exhumation. Crustal Relationship Between Subduction and Magma Genesis. Geoscience Canada, melting is triggered by a variety of processes, including relaxation following 33(4), 145–168. crustal thickening. Melting initiates at the base of the crust, but eventually occurs at shallower crustal levels. Arcs under extension have a steep geothermal gradient and underplating of the crust by mafic magma may transfer sufficient heat to induce anatexis. During a prolonged history of subduction, the dip and location of the subduction zone may vary causing the locus of arc magmatism to migrate and causing intermittent switching from compressional to extensional environments.

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dykes to large plutonic complexes. Sev- SOMMAIRE eral mechanisms facilitate the rise of Le phénomène de déshydratation qui arc magmas. Diapirs rising from a larg- accompagnent la subduction de la er pool of buoyant magma are impor- lithosphère océanique relâche des flu- tant in the ductile lower crust. The ides dans le prisme mantélique sus- rapid rise and the expansion of magma jacent et initie une suite d’événements results in the propagation of fractures, qui conduit à la génération de magmas that facilitate stoping and assimilation qui forment des structures d’îles en arc. in the brittle upper crust. Fracture Les magmas d’îles en arc diffèrent des zones are repeatedly exploited, and the magmas d’autres contextes parce que Igneous Rock Associations net result may be formation of a com- les régimes géothermaux aux lieux de 7. posite batholith. destruction de marges tectoniques sont Water plays an important role différents, étant donné la présence de Arc Magmatism I: Relation- in all stages of arc-magma evolution. fluides et/ou de volatiles issus de la ship Between Subduction Water lowers the temperatures that are déshydratation de la plaque en subduc- and Magma Genesis required for the partial melting in the tion, et du régime tectonique générale- mantle wedge, which produces mafic ment compressif qui inhibe l’ascension J. Brendan Murphy magma, and in the crust, produces fel- du magma, d’où l’importance de l’in- sic magma. Arc magmatism is inti- Dept of Earth Sciences, St. Francis Xavier teraction avec la roche encaissante. En mately related to metamorphism, conséquence, la plupart des magmas University, Antigonish, NS, Canada, although this relationship is complicat- d’îles en arc forment des intrusifs, vari- B2G 2W5, e-mail: [email protected] ed largely because maximum pressures ant des filons-couches, aux dykes aux and temperatures are attained at differ- complexes plutoniques. Plusieurs SUMMARY ent times. Arcs under compression mécanismes favorisent l’ascension des Dehydration of subducted oceanic lith- undergo rapid thickening, followed by magmas d’îles en arc. Ainsi, l’ascen- osphere releases fluid into the overly- erosional or tectonic exhumation. sion de diapirs à partir d’une grande ing mantle wedge and initiates a chain Crustal melting is triggered by a variety accumulation de magma moins dense of events culminating in the generation of processes, including relaxation fol- que l’encaissant est-il un phénomène of magma that rises to form volcanic lowing crustal thickening. Melting initi- important dans la croûte inférieure arcs. Arc magmas are distinct from ates at the base of the crust, but even- ductile. Cette ascension rapide jointe à magmas in other settings because of tually occurs at shallower crustal levels. l’expansion du magma provoque la the different geothermal regimes at Arcs under extension have a steep geo- propagation de fractures, ce qui facilite destructive plate margins, the presence thermal gradient and underplating of l’agrégation et l’assimilation de l’encais- of fluids/volatiles derived from dehy- the crust by mafic magma may transfer sant dans la portion supérieure cas- dration of the subducted slab and the sufficient heat to induce anatexis. Dur- sante de la croûte. Fréquemment, le generally compressive tectonic regime ing a prolonged history of subduction, magma injecte ces zones de fractures, that inhibits the ascent of magma, the dip and location of the subduction ce qui conduit parfois à la formation thereby promoting extensive interac- zone may vary causing the locus of arc d’un batholite composite. tion with the adjacent wall rocks. As a magmatism to migrate and causing L’eau joue un rôle important à result, most arc magmas solidify as intermittent switching from compres- toutes les étapes de l’évolution des intrusive bodies, ranging from sills and sional to extensional environments. magmas d’îles en arc. La présence 146 d’eau abaisse les températures de sented here. The products of arc mag- fusion partielle dans le prisme man- matism, from the pyroclastic and lava télique, ce qui conduit à la formation deposits that erupt at the surface to the de magmas mafiques, et de magmas intrusive rocks in the plumbing system felsiques dans la croûte. Le magma- at depth, are described, as are the vary- TETHYS tisme d’île en arc est intimement lié au ing processes governing the ascent of SEA métamorphisme, bien que cette rela- arc magma and the controversies asso- tion soit compliquée, surtout parce que ciated with the mechanisms of intru- les pressions et les températures maxi- sion. The critical role of water, derived males sont atteintes à des moments from dehydration of the subducted PERMIAN 225 million years ago différents. Les magmas d’îles en arc en slab, is considered in all stages of arc- situation de compression s’épaississent magma evolution, from the generation rapidement, puis sont érodés ou of the magma in the mantle and crust, exhumés tectoniquement. La fusion de during its rise toward the surface, to its la croûte est déclenchée par une variété final cooling and crystallization. This de processus, dont la détente accompa- article concludes with an examination gnant l’épaississement crustal. La of the varying relationships between fusion se produit d’abord à la base de arc magmatism, metamorphism and la croûte, mais elle atteint éventuelle- deformation during subduction-related ment des niveaux moins profonds de la orogenesis. CRETACEOUS 65 million years ago croûte. Les magmas d’île en arc en situation d’extension ont un gradient PLATE TECTONIC SETTING Asia géothermal très pentu, et le placage de Plate tectonics plays a crucial role in N.A. la croûte par un magma mafique peut y the generation of arc magmas. On a Africa apporter assez de chaleur pour provo- globe of constant volume, the creation India quer l’anatexie. Si l’histoire de la sub- of lithosphere at constructive margins S.A. Australia duction se prolonge, le contexte d’un (where two plates move apart) is bal- système d’îles en arc peut alterner anced by the destruction of lithosphere épisodiquement d’un régime en com- at destructive margins (where two Antarctica pression à un régime en extension. plates collide). This balance is apparent PRESENT DAY on paleocontinental reconstructions for Figure 1. Reconstructions over the INTRODUCTION: A REVIEW OF THE the past 200 million years, which show past 200 million years showing how TECTONICS OF DESTRUCTIVE PLATE that the Atlantic Ocean expanded by expansion of the Atlantic Ocean was MARGINS seafloor spreading as the Pacific and accompanied by contraction of the This is the first of two articles on arc Tethys oceans contracted by subduc- Pacific and Tethys oceans. magmatism intended for geoscientists tion (Fig. 1). Subduction eventually who do not specialize in petrology or resulted in closure of the Tethys Ocean a dense oceanic plate beneath an adja- geochemistry, but who are interested in about 35 million years ago as the colli- cent, less dense plate (Figs. 3a, b). A the relationship between the tectonic sion of Africa and India into Eurasia long, narrow, curvilinear trench in the evolution of arcs and magma petrogen- formed the Alpine-Himalayan moun- ocean floor develops where the sub- esis. This article is intended to explore tain chain. However, subduction has ducting plate starts its angled descent that relationship and to set the stage persisted along most of the circum- into the mantle. The deepest trench for a companion article that focuses on Pacific region, where the abundance of occurs in the western Pacific, where the geochemical, isotopic and petrolog- arc-related volcanic activity has result- the Marianas Trough is 11.5 km deep. ical evolution of arc magmas and that ed in this region being called the “Ring As oceanic crust is denser will be published later by this journal. of Fire” (Fig. 2). Petrologic studies than continental crust, subduction A review of the role that plate tecton- from this region provide the founda- preferentially removes the oceanic lith- ics plays, including the effects of sub- tion for our understanding of arc osphere along convergent plate bound- duction on the upper plate (the trench, processes. aries where continental and oceanic the fore-arc, arc and backarc), the con- Although the detailed mecha- crust is juxtaposed. Where two oceanic trasting heat-flow regimes, and the nisms are still a matter of debate, in plates meet at a convergent boundary, effect of changes in the profile of the general terms, subduction of oceanic the older (and therefore denser) ocean- subduction zone with time is first pre- lithosphere is caused by the sinking of ic lithosphere is preferentially subduct- GEOSCIENCE CANADA Volume 33 Number 4 December 2006 147

Figure 2. Modern plate boundaries showing the distribution of volcanoes. Volcanoes on the upper plate near convergent boundaries are subduction-related. Note the concentration of volcanoes around the “Pacific Ring of Fire”. ed. The net effect is that subduction of the subducting lithosphere, and the the chemistry of arc-related magma- efficiently removes old oceanic litho- motion of the upper plate (e.g. Bird tism. sphere. So, despite the fact that the 1988; Gutscher et al. 2000; Gutscher stratigraphic record of marine deposits 2002). The shallower angles of subduc- Volcanic Arcs indicates the existence of oceans as far tion on the eastern Pacific margin are The effect of subduction zones on back as the Early Archean, the oldest generally attributed to the subduction crustal evolution is preserved in the intact oceanic lithosphere preserved in of younger (and therefore more buoy- tectonic evolution of the plate above modern ocean basins is less than 200 ant) oceanic lithosphere, subducting the subduction zone (upper plate) where million years old. Continental crust oceanic plateaus (e.g. Dixon and Farrar there is a relatively predictable zona- has a much greater preservation poten- 1980; Pilger 1981; Murphy et al. 1998; tion (Fig. 3). At some distance from tial than oceanic crust. Continental Gutscher et al. 2000; Yáñez et al. 2001; the trench, an arcuate chain of volca- crust older than 200 million years is Collins 2002) and/or the westerly noes, or volcanic arc, is developed above common, and Archean crust between migration of the North American and the subduction zone, fuelled by magma 3.5 and 4.0 billion years old is pre- South American plates (Bird 1988; rising from above the subducting slab. served in the ancient shield areas of Ramos et al. 2002). Irrespective of the Modern arcs tend to expose several continents (e.g. Bowring and subduction angle, the descending slab the surface or near-surface expression Williams 1999). overwhelmingly consists of oceanic of arc magmas and so are dominated The location of subduction lithosphere. Sediments deposited in the by lava and pyroclastic deposits and zones beneath the surface is defined by trench, however, can also be dragged some feeder dykes and sills. Beneath a an inclined zone of seismic activity, down the subduction zone to consider- volcanic arc, however, are magma which extends to about 700 km depth. able depths. In addition, in some chambers that are a complex labyrinth Around the Pacific “Ring of Fire”, the regions, such as in the Andes of west- of intrusions, which represent the inte- angle of subduction varies from near ern South America, parts of the upper rior of the arc plumbing system (Marsh vertical to subhorizontal, and is gener- plate can be sliced off and dragged 1988). In many ancient arc systems, ally steeper in the western Pacific than down the subduction zone, a process uplift and erosion have removed much in the eastern Pacific (Pilger 1981). known as subduction erosion (Stern 1991). of the volcanic arc, so that the crystal- This angle depends on a number of Upper plate crust can be entrained in lized products of the magma chambers factors, including the age and thickness the subduction zone and contribute to are exposed as large plutonic bodies, 148

typically of granitic to dioritic composition. Therefore, to obtain a compre- hensive view of arc magmatism, it is necessary to combine data from modern and ancient arcs. There are two types of volcanic arcs; i) island arcs (Fig. 3a), which are an arcuate chain of volcanic islands outboard of a continental landmass, like those in the modern western Pacif- ic Ocean, which are situated in the overriding upper plate above the subduction zone; and ii) continental arcs (Fig. 3b) like the modern Andes, which are a chain of active volcanoes on the edge of a continental landmass. Island arcs may be subdivided depending on whether they are capped by a mafic crust, like the Philippines, or by continental crust, like Japan or northern New Zealand (Fig. 3a). This distinction has a profound affect on the petrological evolution of the arc magmas (Gill 2001). Island arcs capped by mafic crust tend to be dominated by mafic to intermediate magmas; those capped by continental crust have a higher proportion of intermediate to felsic lavas, and are composition- ally similar to continental arcs (e.g. Miyashiro 1974). Although island arcs are typically 200-300 km wide and up to 2000 km long, volcanism at any one time is typically concentrated in a region about 10 km wide, and about 110 km above the subduction zone. However, as the dip of the subduction zone varies over its lifespan, so too does the location of the volcanism. Figure 3. Styles of subduction at a convergent plate boundary. Open arrows indi- Over a long period of time, therefore, cate motion vectors of the descending plate. a) Subduction of denser (and older) a broad band of arc-volcanic rocks can oceanic lithosphere beneath less dense lithosphere to create an island arc. The less be produced, even though only a rela- dense lithosphere may consist of younger oceanic lithosphere (e.g. Marianas) or tively narrow band is active at any one may be capped by continental crust (e.g. Japan). Roll-back of the subduction zone time. may create a backarc basin. b) Subduction of oceanic lithosphere beneath a conti- Also, island arcs are segment- nental margin (e.g. Andes). In both scenarios, dehydration reactions cause ed into belts up to 300 km long, which blueschist and eclogite facies to form at depth in the vicinity of the subducted slab. are terminated by deep-penetrating Water released from these reactions invades the overlying mantle wedge causing fractures that accommodate along- melting in the mantle. Rising mafic magmas may underplate the crust (modified strike variations in the dip of the sub- after Tatsumi and Eggins 1995). (SCLM - Sub-Continental Lithospheric Mantle) duction zone. Magmatism in the vicinity of these fractures has chemical and isotopic features that are different from typical arc magmas, and may GEOSCIENCE CANADA Volume 33 Number 4 December 2006 149 reflect local upwelling of underlying erupted from either the central vent or onto the inner trench wall to form an asthenosphere (e.g. Marsh 1979, 1982; along the flanks of the volcano. This accretionary prism. As the accretionary Kay et al. 1982). Other patterns of arc alternation reflects the varying style of prism grows, the trench migrates, volcanism are rarer. For example, a eruption over time and is a characteris- which in turn can result in migration of secondary narrow chain of volcanoes tic of the Cascade Range, in the Pacific the locus of arc magmatism. Alterna- may occur about 175 km above the Northwest. Calderas up to 100 km tively, fore-arc material can be trans- subduction zone (Marsh 1979, Tatsumi across are found at the summits of ferred from the overriding plate to the and Eggins 1995). these volcanoes. The volume of pyro- subducting plate by subduction erosion clastic material associated with a single and may then be transported down the Products of Arc Magmatism eruption can be substantial and its subduction zone (cf. Stern 1991, 2002). Because of their lower viscosity, erup- effects can be global. In 1991, the Subduction erosion occurs on more tions of mafic lavas are generally not eruption of Mount Pinatubo in the than half of the modern convergent explosive and produce broad volcanic Philippines jettisoned an estimated 5 plate boundaries, especially where the structures that have gentle slopes. cubic kilometres of debris into the overriding plate is moving rapidly Intermediate and felsic magmas in vol- atmosphere, 5 times more than the towards the subduction zone and the canic arcs, however, are viscous, and volume of debris associated with the width of fore-arc in these regions is move so slowly that most of the 1980 eruption of Mount St. Helens. An less than that predicted by the dip of magma cools underground forming eruption in the vicinity of Lake Taupo, the subduction zone. High pressure, large plutons of granite and diorite (e.g. New Zealand in 1881, is estimated to coesite- (e.g. Schreyer 1988) and dia- Clarke 1992, Pitcher 1993). As viscous have ejected between 30-50 cubic kilo- mond-bearing (Xu et al. 1992) meta- felsic magma rises toward the surface, metres of debris in just a few minutes. morphic rocks in subduction zone the reduced pressure results in the for- Some pre-historic eruptions are esti- complexes indicate that crustal material mation of gas bubbles that eventually mated to have ejected as much as 2500 can be subducted to depths of at least cause the magma to explode into hot cubic kilometres of debris and may 100 km. The preservation of high pres- fragments (Cashman et al. 2000). have had significant impact on climate. sure minerals in low-grade metamor- Where this magma does extrude, its phic rocks requires more rapid exhu- high gas content usually causes violent Fore-aarc mation of arcs than the relatively slow explosions, which produce steep-sided The region between the volcanic arc erosion-controlled exhumation, and is volcanoes that eject far more airborne and the trench is known as the fore-arc generally interpreted to reflect tectonic fragments than lava, and result in pyro- (or “arc-trench gap”, Fig. 3a), the width thinning processes (e.g. Brown 1993). clastic deposits such as volcanic breccia, of which depends on the dip of the Additional evidence that subducted tuff and ash. Such explosive activity subduction zone. In steeply dipping material is recycled by the subduction propels mixtures of gases, or aerosols, subduction zones, the fore-arc is nar- zone processes is provided by the together with fragments of hot magma row, whereas in more shallowly dip- chemical and isotopic composition of and wall-rock into the air. When this ping subduction zones, the fore-arc is arc magmas (Murphy 2007). mixture becomes too dense to be vent- wide. The fore-arc, therefore, is gen- ed upward, it collapses under its own erally much narrower in Western Pacif- Backarc weight, producing a rapidly moving, ic, compared to Eastern Pacific, arc Although the precise mechanisms are ground-hugging avalanche known as a systems. This relationship is generally controversial, the region behind the arc pyroclastic flow that can spread as far as interpreted to indicate that the subduc- may be either in extension or compres- 100 kilometres from the site of the tion zones must attain a certain depth sion. There may be several mecha- eruption (Sparks et al. 1973). Because (about 110 km) before processes in nisms responsible for backarc exten- of their explosive character and high them trigger the generation of arc mag- sion, but the currently popular model velocity (typically between 50 and 200 mas. The frontal part of the fore-arc is the roll-back model (Dewey 1980). This km/hr), pyroclastic flows have claimed may contain pyroclastic deposits, lavas model proposes that as the disconnect- many lives in recorded history. or immature volcaniclastic sediments, ed end of the dense slab collapses into In contrast to the gentle all derived from the active part of the the asthenosphere, it may “roll-back” slopes associated with mafic volcanoes, arc, which rest upon a basement of oceanward (Fig. 4). The retreat of the eruption of intermediate to felsic lavas igneous and metamorphic rocks. Near slab is associated with seaward migra- produces dramatic volcanic edifices, the trench, the fore-arc exhibits com- tion of the trench and causes extension including composite volcanoes, which con- plex deformation as sediments deposit- in the upper plate. Therefore, subduc- sist of alternating layers of pyroclastic ed in the trench are scraped off the tion zones may move appreciably over deposits and lava flows that may be downgoing plate and are plastered timescales of 100 million years relative 150 to some other reference frame. Sub- ducting slabs sink more steeply than the inclinations of Benioff seismic zones, which mark transient positions, not trajectories, of slabs. The convergent margin can move faster than the roll-back if the upper plate is also moving over the descending one, as in the case of the westward drift of North America since the break-up of Pan- gaea. Episodic subduction occurred along the western margin of North America over the past 100 million years, demonstrating significant motion of subduction zones. Roll-back is most effective where old oceanic lithosphere is subducted, and extension in the upper plate results in backarc spreading, in which a backarc basin (also known as a marginal basin) opens up behind the arc. This style of subduction predominates in the western Pacific Ocean, where backarc basins separate island arcs from the Asian continent. For example, the Sea of Japan and the West Philippine Basin are backarc basins that separate the island arcs of Japan and the Philippines from the Asian mainland. As these basins are floored by oceanic crust, it is not surprising that the pillow lavas produced are similar mineralogically and texturally to mid-ocean ridge basalts (MORB). Backarc basins, however, lack the clas- sical, well-defined magnetic anomaly

Figure 4. Diagrams illustrating how arcs switch from compressional to extensional tectonic settings (Collins 2002). Roll-back of the subduction zone results in the generation of an inter-arc rift (A) followed by a backarc basin (B). Eventually, the subduction zone overrides an oceanic plateau (B to C) which results in shallowing of the subduction zone and backarc basins formed by extension become inverted. After the oceanic plateau is completely overridden, more angled subduction, roll-back and inter-arc basin development resumes (D). GEOSCIENCE CANADA Volume 33 Number 4 December 2006 151 patterns of MORB, possibly because may migrate (shallow or steepen) as the together, backarc basins may well be the rifting is asymmetric and the mag- age or thickness of the subducted lith- the sites where magmatism and high- matism is diffuse (e.g. Uyeda 1978). osphere, or as the relative motion of grade metamorphism develop more According to Collins (2002), extension the converging plates, changes (e.g. readily than in adjacent, drier rocks, and decompression associated with Molnar and Atwater 1978; Hynes and and so may exert a fundamental con- roll-back may result in asthenospheric Mott 1985; Ramos and Aleman 2000; trol on the petrology of magmas pro- upwelling and voluminous magmatism Gutschner et al. 2002). As the dip and duced in arc systems (Collins 2002; in the overlying mantle and crust. In location of a subduction zone change, Hyndman et al. 2005). In support of most cases, geochemical and isotopic so too do the width of the arc-trench this model, Collins (2002) points out investigations reveal the influence of a gap and the locations of the volcanic that the mineralogy of many granulite nearby arc on the composition of the arc and the backarc basin. If plate con- facies complexes requires geothermal mantle source. vergence is high and young, and/or gradients between 20° and 50°C per In the geological record, ves- buoyant oceanic crust is being subduct- km, which is much higher than gradi- tiges of ancient backarc basins also ed, the subduction zone becomes sub- ents in collisional environments, but is may be distinguished from mid-ocean horizontal, and is too shallow to gener- consistent with those expected in ridge settings by the character of the ate arc magmas (Dickinson and Snyder backarc settings. sedimentary strata associated with the 1979; Pilger 1981; Bird 1988; Ramos et Backarc regions in continental basalts. As backarc basins fringe vol- al. 2002; English et al. 2003). Such an environments may alternate between canic arcs on one side and a continen- environment occurs in several seg- periods of extension and compression tal land mass on the other, they may ments of the modern Andes (Fig. 5). (known as “tectonic switching”, Collins contain abundant volcaniclastic sedi- Although much of the Andes is a clas- 2002). Backarc basins that result from ments that are cannibalized from the sic continental arc, there are narrow extension induced by periods of roll- adjacent arc or clastic sediments segments where no modern magma- back and slab retreat can intermittently derived from the continental hinter- tism occurs. Seismic images reveal that become inverted and polydeformed by land. In contrast, most mature mid- beneath these segments the subduction compression, induced by flat-slab sub- ocean ridge settings are sediment- zones are sub-horizontal. Pilger (1981) duction related to arrival at the trench starved; consequently, chemical (e.g. drew attention to the correspondence of an oceanic plateau (Fig. 4). During chert) and pelagic sediments predomi- between these zones and the subduc- the extensional phase, the high geot- nate. tion of thick and buoyant oceanic hermal gradient and ascending mafic For continental arcs with plateau (Fig. 5). magma induce crustal melting and backarcs in extension, regions of Backarc regions in compres- high-grade (granulite-facies) metamor- thinned continental crust are produced sion occur predominantly in continen- phism. in which the rise of hot mantle towards tal settings where compression of a In a general sense, subduction the surface generates crustal melts as thermally softened crust can lead to zones, and therefore arc magmatism, well as melts from the decompressed polyphase deformation and both thick- can last for hundreds of millions of mantle. For example, in the Taupo skinned and thin-skinned fold-and- years. The processes that result from Volcanic Zone of northern New thrust belts (Ramos and Aleman 2000). prolonged subduction profoundly Zealand, the crust is as little as 5 km Much of the backarc region of the affect the composition, thickness and thick with partially molten material, Andes is currently in compression and strength of the lithosphere. Uplift asso- thought to be the source of the is undergoing active deformation. ciated with this tectonothermal activity magma, immediately below (Cole et al. Continental backarcs are characterized is not continuous, but instead appears 1998, 2005). Magmas in continental by thin hot lithosphere and have Moho to be episodic. For example, although backarcs are commonly bimodal in temperatures of ca. 850°C and lithos- subduction and magmatism have per- composition (i.e. they are dominated pheric thickness of ca. 55 km, com- sisted along the western margin of by basalt and rhyolite, with few “inter- pared to ca. 450°C and 150-300 km, South America for most of the last 200 mediate” rocks) and are associated respectively, for adjacent cratons (Fig. million years, it was only about 15 mil- with high temperature–low pressure 6, Hyndman et al. 2005). Continental lion years ago that the rate of uplift metamorphic belts and possibly meta- backarcs are hotter, and therefore, an was sufficient to reverse the flow of morphic core complexes (e.g. Hynd- order of magnitude weaker than adja- the Amazon river from its ancestral man et al. 2005). cent lithosphere. In addition, these westward path into the Pacific Ocean basins may contain thick deposits of to its current eastward path into the Profile of the Subduction Zone sediments that are fertile material for Atlantic Ocean (e.g. Hoorn et al. 1995). Subduction zones are not static; they the formation of crustal melts. Taken Many geoscientists attribute this rapid 152

Heat Flow 90oW Carib. Plate 70o Cocos Choco Bl. The subducting slab, the region of arc 1.5 cm/a magma generation, and the varied Plate components of backarc regions can all 8.9 cm/a be detected by surface heat flow meas- 6.0 cm/a Eastern urements across oceanic and continen- Cordillera tal arc systems (Fig. 6). Heat flow is low, both in the vicinity of the trench and in the arc-trench gap (fore-arc), South reflecting the descent of cold oceanic 0o lithosphere into the mantle and the American absence of convecting mantle between Carnegie Ridge the fore-arc and the subducting plate. 7.0 cm/a Plate The effect is similar to that of immers- Inca ing an ice cube into hot tea. For the Plateau time it takes for the heat to be transferred from the tea into the ice, the ice remains anomalously cold. Similarly, Brazilian the temperature in, and adjacent to, the Pacific subduction zone is lower than the sur- Shield rounding mantle at the same depth. As Ocean a result, the geothermal gradient (the increase in temperature with depth), o 7.8 cm/a 15 S and therefore, the heat flow to the surface, are low in the vicinity of subduction zones. The geothermal gradient can be as low as 10°C/km, and the Nazca heat flow as low as 0.05 W/m2. In contrast, the region beneath Plate volcanic arcs is dominated by high heat flow. Volcanic activity accounts for about 10% of this anomaly and the remainder is attributed to the ascent of hot magma towards the surface and/or mantle convection (Gill 1981). As the region below the arc is hotter than the 8.4 cm/a 30o surrounding rocks, heat flow measure- 2 Sierra ments are high (up to 0.1 W/m ) and Pampeanas the geothermal gradient is steep (typically 30°C/km). Until recently, it was thought that heat flow in the backarc depended Figure 5. Andean arc magmatism, and the correspondence between sub-horizontal on whether it is under extension (high subduction zones and the overriding of oceanic plateau (see Pilger 1981; Yáñez et heat flow) or compression (low heat al. 2001; Ramos et al. 2002). The bars indicate regions where no active volcanism flow). Extensional backarcs are hot, occurs. These regions are above subhorizontal subduction zones. irrespective of whether they are floored by oceanic (e.g. Western Pacific) or continental (e.g. Basin and Range) uplift to the effect of higher rates of (Ramos and Aleman 2000). Arc mag- crust (Wiens and Smith 2003). Howev- compression between the converging matism can be pre-kinematic, syn-kine- er, Hyndman et al. (2005) point out South American and Pacific plates on a matic and post-kinematic with respect that many modern non-extensional crust that was thermally weakened by to discrete deformational events. backarcs have heat flow values that are prolonged arc-related magmatism comparable with the region beneath GEOSCIENCE CANADA Volume 33 Number 4 December 2006 153

dense than the surrounding rock, particularly in the lower crust which is more ductile than the upper crust. Experiments show that magma may attain enough buoyancy to rise from large magma chambers as tear-drop shaped bodies (diapirs) with regular spacing between them (Fig. 7; Marsh 1982; Miller and Paterson 1999; John- son et al. 2001) or as nested diapirs (Paterson and Vernon 1995). For example, volcanic centres in the Cen- tral American arc are typically spaced about 25 km apart, and are thought to be the result of diapiric uprise from a larger pool of magma that parallels the length of the arc (Marsh 1979; Gill 1981). Although field observations indicate that diapirs can arch the overlying layers, most petrologists contend Figure 6. Heat flow profiles across the Cordilleran volcanic arc showing high heat that this process acting alone is unlike- flow in the backarc region (after Hyndman et al. 2005). ly to enable magma to rise into the shallow brittle crust (Hutton 1988; volcanic arcs, and are twice that of the because widening fractures create space Pitcher 1993). The rise of a diapir is adjacent cratons (0.04 W/m2, Chapman for rising magma to exploit. self-limiting because its buoyancy and Furlong 1992). The reason for the decreases as it rises (the density of high heat flow in such backarcs is Mechanisms of Intrusion rocks surrounding the magma decreas- unclear. However, as many geodynam- In arc environments, vast volumes of es as the surface is approached and ic models treat shortening as being magma rise toward the surface, and magma becomes denser as it cools). geologically instantaneous (e.g. Eng- this magma must occupy space that An alternative mechanism for land and Thompson 1986), it is possi- was previously occupied by solid rock. the upper crust proposes that magma ble that some of these backarcs may Unlike continental rifts, in which rises by assimilating the overlying have inherited the high heat flow from regional crustal extension provides rocks. The occurrence of partially a recent history of extension (from space for ascending magma, in arc digested xenoliths in many arc-related which the heat has yet to dissipate) and environments the “room problem” intrusive rocks lends support to this switched to compression by the arrival (see Paterson et al. 1991) is still contro- hypothesis. However, most petrologists of a microcontinent or oceanic plateau versial after more than a century of are still swayed by Bowen’s (1928) ele- at the subduction zone. debate. Although, to some extent, this gant arguments that magmas do not space problem applies to all tectonic have sufficient heat content to digest ASCENT OF ARC MAGMA settings, it is particularly acute in arc large quantities of host rock, and the Three fundamental properties influ- settings where compressional tectonics loss of energy in the digestion process ence the rate at which arc magma predominates, and crustal thickening causes the magma to cool and crystal- ascends: i) magma is less dense than impedes the ascent of magmas. The lize, rather than rise. Geochemical the wall rocks and is, therefore, buoy- less efficient ascent of magmas in arc and isotopic data from many plutons ant, ii) a magma with lower viscosity settings has important petrological also argue against significant assimila- rises with greater ease than a more vis- consequences because such environ- tion of local wall rocks (e.g. Glazner cous magma (therefore, mafic magma ments promote extensive magma-wall- and Bartley 2006). generally rises more readily than felsic rock interaction and cooling, factors Magma may be able to create magma, which is likely to be trapped in that explain the abundance of plutonic and exploit cracks and fractures in the magma chambers where it cools to rocks in arc environments. overlying crust. Experiments show that form a pluton), and iii) magma rises Where sufficient magma is melting results in a 15 percent increase more rapidly in an extensional environ- produced (> 15%), it congregates and in volume, and this expansion may ment than in a compressional setting attains buoyancy because it is less produce enough pressure to fracture 154

Field observations demon- strate a close relationship between igneous intrusions and adjacent faults (Fig. 7) suggesting that magma exploits large-scale crustal weaknesses (e.g. Sawyer 1994). Fractures associated with faulting may, therefore, be favourable locations for magma emplacement (Fitch 1972) and significant volumes of magma can migrate through them (e.g. Clemens 1998; Brown and Solar 1998). A magma intruded into an extensional tectonic environment, for example, can exploit opening fractures that create space for it. Extensional environments can occur locally in arc settings, especially where a fault within the arc bends or jogs (e.g. Guineberteau et al. 1987; Hutton 1988; Murphy and Hynes 1990; Kouk- ouvelas et al. 2002; Fig. 7) and magma ascent is especially rapid in such environments.

Extensional Regimes A number of recent studies on modern arcs have indicated the importance of extensional regimes in the generation Figure 7. The ascent of arc magma (modified after Pitcher 1993). By (A) of arc magmas (e.g. Hyndman et al. diapirism, (B) exploiting faults, (C) stoping involves the foundering of wall rock 2005) and that the composition and into the magma chamber, (D) exploitation of structures such as local extension or ascent of these magmas are profoundly pull-apart faults. These mechanisms may work in concert as rising diapirs of influenced by faults and fractures magma may exploit zones of crustal weakness, such as faults and extensional zones whose orientation reflects plate bound- in those faults. In these zones, wall rocks are weakened, promoting stoping. One of ary conditions (Tibaldi 1992; Spinks et the main features of stoping is the presence of abundant xenoliths (see inset). al. 2005). For example, Tibaldi (1992) shows that these environments gener- the overlying rock and allow the 1982). Evidence supporting this ate wide belts of magmatism, and magma to rise. The presence of water process includes the occurrence of Spinks et al. (2005) show that the in the arc magmas enhances its ability large xenoliths in plutonic rocks. regions with the greatest extension in to create fractures (see Downey and However, it is unclear how important the Taupo Volcanic Zone of New Lentz 2006). Magma will also increase this process is in facilitating the ascent Zealand produce active calderas up to in volume if it rises sufficiently fast of magma over long distances (see 50 km wide that are dominated by fel- that it does not lose much heat (i.e. Clarke et al. 1998). In addition to the sic magmatism, whereas areas with the adiabatically). The resulting expansion lack of geochemical and isotopic evi- greatest transtension produce less of the magma may also result in frac- dence for significant assimilation, the active andesitic stratovolcanoes. The ture propagation that weakens the fact that xenoliths are volumetrically implication is that these varying struc- resistance of overlying rock. small, even on pluton floors and that tures provide conduits to magmas The foundering of overlying large plutons grow incrementally from ascending from different structural lev- blocks into the magma, (Fig. 7), called small batches of magma (blocks can els. Paterson and Tobsich (1992) and stoping, is enhanced by the tendency for only fall through liquid magma) togeth- Petford et al. (2000) show that strain magma to invade fractures in the host er suggest that stoping is not a signifi- rate along these structures influences rocks and pry them loose so that they cant method of pluton emplacement magma composition and eruption style. drop into the magma (e.g. Marsh (Glazner and Bartley 2006). Several studies emphasize the GEOSCIENCE CANADA Volume 33 Number 4 December 2006 155 importance of intra-arc strike-slip tec- locally during the later stages. et al. 1994). It is a general principle of tonics in controlling the rise of mag- It appears that several mecha- igneous petrology that water lowers mas in modern arcs (e.g. central Andes: nisms may act in concert to facilitate solidus temperatures of the mantle. So De Silva 1989; NE Japan: Sato 1994; the rise of arc magmas. Many diapirs the relentless invasion of water into the Ecuador: Tibaldi and Ferrari 1990; are located near fault zones and appear mantle wedge promotes melting and Mexico: Tibaldi 1992; Kamchatka: to have exploited zones of crustal the generation of arc magmas. The Baluyev and Perepelov 1988). This is weakness. Magma exploiting such presence of water affects the stability similar in principle to example D in zones may rapidly rise and expand, of silicate and accessory minerals at the Figure 7, where the strike-slip fault resulting in fracture propagation, facili- site of melting and during subsequent occurs within an arc setting. Magma- tating stoping and assimilation. As cooling. Water content influences tism in the Kamchatka Volcanic Belt, fracture zones are repeatedly exploited, magma composition, chemical evolu- for example, is associated with regional the net result may be a composite tion and the kinetics of mineral transcurrent faults that reach a depth batholith that typifies arc systems. growth. The combination of these of 40 km and it is suggested that the effects can be recognized in the miner- depth of magma generation is con- EFFECT OF WATER alogy, textures, geochemistry and iso- trolled by these faults (Baluyev and Perhaps the most unifying characteris- topic signature of arc magmas. Perepelov 1988). Tibaldi (1992) attrib- tic of arc magmas is the important role Even a small amount of water utes the obliquity between the Mexican played by water in all stages of magma lowers the melting temperature (or the Volcanic Belt and the Middle Ameri- evolution, including magma genesis, solidus) of rocks (Fig. 8). Common can Trench to transcurrent motion ascent toward the surface, cooling and mantle minerals such as garnet, olivine, along adjacent faults. According to crystallization, and during volcanic orthopyroxene and clinopyroxene do Ferrari et al. (1989), deep penetrating eruptions. The explosive nature of arc not readily admit water into their crys- faults in intra-arc environments create volcanic eruptions, the abundance of tal structure (this tendency is reflected favourable conditions for rapid ascent vesiculated products such as pumice by the lack of water in their mineral of mafic magmas with little crustal and phenocrysts of hydrous mineral formulae). Because the water molecule contamination. phases are all indicative of magmas is polar, the negative end of the mole- with high volatile content. cule tugs on the cations of minerals, Batholiths weakening their bonds. The weakened Field studies of large batholiths in arc Water in the Mantle Wedge crystal structure requires less energy to regimes show that they represent the The subducted slab of oceanic litho- break the bonds, and so magma can vestiges of voluminous magma cham- sphere contains abundant water, and as form at a lower temperature than bers in the middle and upper crust (e.g. it descends into the mantle it is under dry conditions. This effect is Clarke 1992; Pitcher 1993; Coleman et warmed and undergoes prograde dehy- especially profound in the mantle, al. 2004; Miller and Paterson 2001; dration reactions. According to Tatsu- where dry rocks are resistant to melting Vigneresse and Bouchez 1997) that mi and Eggins (1995), the release of because the elevated pressure reduces were constructed incrementally by water from prograde dehydration reac- bond lengths and favours more dense repeated injection from elongated tions occurring in the slab (amphibole- mineral structures. However, at these dykes (Hutton 1992; Miller and Pater- chlorite assemblage at 110 km depth, pressures, water’s ability to weaken son 2001) or were emplaced as nested phlogopite at 175 km depth) may be crystal structures becomes even diapirs (Allen 1992; Paterson and Ver- responsible for two parallel volcanic greater, so that wet rocks become less non 1995). These batholiths, therefore, arcs. The released water is a supercriti- resistant to melting with increasing are composite and have numerous cal fluid under mantle conditions, and depth. internal contacts that vary from sharp has the capacity to extract soluble ele- In addition to weakening the and planar to gradational and irregular. ments from the slab (e.g. K, Rb, Ba, Sr, crystal structures, water ascending A recent study of the Tuolumne U, Pb, Th and Cs, known as the large from depth is a heat flux because it Batholith of the Sierra Nevada (Zák9 ion lithophile elements or LIL), which rises rapidly and does not lose much and Paterson 2005) proposes that its are transported to, and become heat to its surroundings. The combina- evolution involved fractional crystal- enriched in, the overlying mantle tion of weakened mineral structures lization, magma mixing on a kilometre- wedge (e.g. Spandler et al. 2003; Leib- and additional heat promotes melting scale, stoping along sharp contacts, and scher 2004). and the formation of magma in the downward return flow (and/or margin The fluid that invades the overlying mantle wedge and in the collapse) of older magma units, with mantle wedge plays a pivotal role in crust. dyke injections becoming important generating arc magma (Fig. 3, Peacock 156

mal-isobaric diagrams illustrate the importance of understanding fluid- mineral interaction (Fig. 9; modified after Eggler 1978; Thompson 1983; Murphy 1989). These diagrams focus on the relationship between non- volatile phases (A), hydrous phases (H) and carbonate phases (C). Hydration reactions are represented on the AH join, carbonatization reactions on the AC join, and fluid composition on the CH join (Fig. 9a). Mantle rocks may contain several hydrous and carbonate phases (e.g. phlogopite, amphibole, calcite, and dolomite) with varying degrees of hydration and carbonatization. For simplicity, individual hydrate and carbonate phases are not displayed as they do not affect the following arguments. The stippled triangle in Figure 9a represents an assemblage with no free fluid phase (i.e. volatile- absent, see Thompson 1983). For bulk compositions within that triangle, fluids injected become absorbed in Figure 8. The effect of water on magma genesis. Phase diagram emphasizing the hydrate (H) or carbonate (C) phases. role of water in magma source rock (modified from Winter 2001) showing a com- With increasing temperature, the reac- parison between the dry and water-saturated solidi for granite, basalt and peridotite, tion and typical shield and oceanic geotherms. The breakdown curves for hydrous phas- C+H 6 A+fluid es in the mantle such as amphibole and phlogopite are shown. occurs and anhydrous phases exist in equilibrium with a fluid (as shown in Potential Significance of Mineral- content resulting in silica-saturated Fig. 9b). Fluid Interaction in the Mantle basalts, and higher CO2 content leading Incremental input of a CO - Wedge to the generation of silica-undersaturat- 2 rich fluid (G, Fig. 9a) into a rock of In the mantle wedge, fluid rising from ed basalts. In addition, fluids derived bulk composition J would result in the subduction zone interacts with from the dewatering of the slab invade progressive dissolution of anhydrous minerals and fluids already present. and react with minerals in the mantle phases (path 1, Fig. 9a). If the ratio of Thus, the fluid composition at the site wedge, promoting the formation of hydrate to carbonate phases in the host of melting probably has components minerals that permit water into their rock is less than the CO /H O ratio of of both the invading fluid from the crystal structure, such as hornblende 2 2 subduction zone, the indigenous fluid and phlogopite. For the most part, the fluid, then dissolution of the previously added to the mantle wedge, these fluids migrate along grain bound- hydrate-bearing phases (H) will occur and fluid released by mantle minerals. aries. The resulting solution and pre- via the continuous reaction: In addition, some elements are released cipitation of mantle minerals affects A+H+G (fluid) X C from the mineral into the fluid but the bulk partitioning of trace elements A free fluid phase would only other elements present in the fluid may between mantle minerals, fluid and the occur after expiration of A (X, Fig. 9a). be incorporated in a solid phase. melt. Further input of fluid G would result Several experiments have These findings suggest that an in continued precipitation of C and demonstrated that the composition of understanding of the factors that affect dissolution of H (path X to Y). The igneous melts is profoundly influenced this fluid composition is essential. fluid produced by this reaction is by fluid composition at the site of Although these factors are poorly buffered (at constant pressure and melting (e.g. Mysen and Boettcher understood, generalized phase relation- temperature) to constant composition ships shown schematically on isother- 1975; Boettcher 1977) with higher H2O P, which is different from the compo- GEOSCIENCE CANADA Volume 33 Number 4 December 2006 157

CO sition of the invading fluid. This con- 2 tinuous reaction is: H+G (fluid) X C + P(fluid) G i.e. fluid G is spontaneously converted to fluid P. CV (a) Note that in this case the reaction may be irreversible if fluid P is lost (e.g. through partitioning into magma). If fluid loss occurs when the bulk composition is at R, then the bulk CARB composition is deflected from path 1 along path 2 which projects through R C P to P (i.e. a path of constant C/H). If Y the bulk composition continues along R path 1, then buffering of the fluid X CHV phase to composition P is lost when the hydrate phase expires (Y, path 1). ACH At this stage, magma of different com- HV position may be produced. J Input of CO -rich fluid (G, F 2 Fig. 9b) to the higher temperature assemblage (e.g. S) initially causes dis- A H H 0 solution of H (path 1) and precipita- 2 tion of A as the fluid is buffered to ANHYD HYD constant composition M via the continuous reaction: CO2 H + G (fluid) X A + M(fluid) Buffering of the fluid phase would G continue until H is consumed (at h, Path 1). Continued addition of fluid G would eventually lead to renewal of CV (b) buffering at the first appearance of a e N carbonate phase C at d (path 1). Fluid CARB Figure 9. The effect of the introduc- C tion of fluids into a potential source d rock (see Murphy 1989). Schematic AC representation depicting phase rela- M tionships between a volatile-absent N phase A, a hydrous phase H, a carbonate phase C and fluid. Modified after h Eggler (1978). The stippled triangle in (a) represents the fluid-absent region AV S F (see Thompson 1983) and the stippled triangles in (b) are zones of invariant vapour composition (Eggler 1978). AHM HV Within the stippled triangles of (b) A H H 0 fluid composition is held constant dur- 2 ing influx of exotic fluid by re-adjust- ANHYD HYD ment in the modal abundances of the phases at the apices of the composi- HYDRATION tional triangle. 158 loss, at an early stage along path 1, related magmas. This is an important attain the buoyancy to rise to higher results in precipitation of A and H chemical fingerprint for distinguishing levels in the crust. However, reactions (between S and h), precipitation of A among magmas of different tectonic involving the breakdown of biotite (between h and d) or precipitation of A environments and will be discussed in (Fig. 10b; generally between 750 and and C (between d and e). Where partial a later publication (Murphy 2007). 780°C) may generate more magma (up melting occurs, the fluid extracted by to 60%, Clarke 1992), leaving a residue the magma would be significantly more Partial Melting of the Crust in Conti- of granulite that is typical of arc envi- enriched in H2O. Extraction of a fluid nental Arcs ronments (Collins 2002). F from bulk composition S (path 2, Volatile components such as water and If no water is added, (e.g. Fig. 9b), would result in continued pre- carbon dioxide have a strong tendency Spear et al. 1999), then reactions that cipitation of A and dissolution of H. to enter magmas generated in both the generate magma require higher temper- Situations in which buffering is either atures to proceed as a decreases, mantle and the crust. In many tectonic H2O achieved or lost involve sudden settings, the partitioning of water into but in arc settings, the supply of water changes in the composition of the magma reduces P in the source may be continuously replenished H2O fluid, and therefore, may promote rock, thereby raising the solidus tem- (either directly or indirectly) by ongo- enhanced dissolution or precipitation perature and limiting the amount of ing dehydration of the subducting slab. of mantle minerals (depending on how As a consequence, a may remain melt produced. This effect is schemat- H2O the composition of the fluid changes). ically shown in a simplified phase dia- close to unity, and anatexis of crustal

gram in the system K2O-Na2O-Al2O3- rocks in arc environments generally Partial Melting in the Mantle proceeds at lower temperatures, and is SiO2-H2O (KNASH, after Thompson Wedge and Algor 1977, Fig. 10a). The reac- more voluminous than in continental Although the introduction of water tions depicted (Fig. 10) involve the rifts, and spans longer periods of geo- lowers solidus temperatures, there is breakdown of muscovite during the logic time, routinely tens of millions of never enough energy to completely melting of pelites. Reactions 2 and 5 years. melt the mantle wedge. Instead the define the curve for minimum melting mantle undergoes partial melting. of pelite and the formation of felsic Water and the Ascent of Magma Although partial melting of the mantle magma under water-saturated condi- Although the details are controversial, typically produces basaltic magma, in tions (P = P ). In some tectonic water probably plays an important role detail, the overall chemical composi- H2O total in the ascent of magmas, because it environments (e.g. continental rifts), tion of the basalt (especially the trace reduces the viscosity of the melt (e.g. the condition P < P may develop element and isotopic content) is pro- H2O total Mysen 1988). Furthermore, water pres- in the pelites such that the activity of foundly influenced by the relative sta- sure also promotes fracturing in over- water is less than unity (a < 1). In bility of minerals. The crystal structures H2O lying rocks, particularly in the crust. In of some minerals are more weakened this scenario, the univariant reactions the deviatoric stress field that is typical than others and so some minerals are involving water become divariant and of arc regimes, water pressure reduces progressively migrate as a is more prone to melting than others. H2O the effective stresses, which induces Partial melting, therefore, proceeds in a reduced, so that the stability field of fracturing, as demonstrated by classic regular fashion, and melt compositions the assemblage containing water is Mohr circle analysis. As water pressure can be predicted by phase equilibria. expanded. Reactions 2, 3 and 5 move builds up, the Mohr circle migrates For example, at high mantle pressures, to the right, reactions 1 and 4 to the towards the left as effective pressures garnet is more stable than olivine, and left, but reaction 6, which is the fluid- are reduced, until the failure envelope so garnet stays behind in the residue, absent reaction is unchanged. Since is intersected. At this point fractures the invariant point for a given a is and the basalts produced are depleted H2O develop in the host rock, which facili- in the components of garnet relative to the intersection of the univariant tate the rise of magma and fluid (Fig. olivine. A hydrated mantle wedge curves, the invariant point slides along 11). This process is cyclic, as water enhances the stability of several acces- the vapour-absent curve (V) towards pressure builds up and is then released higher P and T as a decreases (see sory minerals (e.g. zircon, titanite, and H2O when fracturing occurs (e.g. Sibson ilmenite). Because several important Thompson and Algor 1977). The net 1987). trace elements and REE elements are effect is that the temperature required for melting pelites increases as a concentrated in these phases, their sta- H2O Cooling History and Stability of Min- bility in a hydrated mantle means that decreases causing the amount of melt erals arc magmas are depleted in these ele- generated to be small (< 10%), i.e. The presence of water also affects the ments relative to, for example, rift- below the critical volume required to sequence of crystallization in mafic GEOSCIENCE CANADA Volume 33 Number 4 December 2006 159

Figure 10a. Schematic phase diagram (modified after Thompson and Algor 1977) showing the melting of pelites in the system

K2O-Na2O-Al2O3-SiO2-H2O (KNASH). Reactions are distributed around the invariant point (I3) according to Schreinmakers rules (see Yardley 1989, p. 217-222). For simplicity, SiO2 is assumed to be in excess, so that the Qtz-absent curve is excluded.

With decreasing activity of water, I3 migrates up the vapour absent curve (V) in the KNASH system. magmas. Water content also affects oxidized magmas, magnetite is an early sitions are characterized by relatively stability of hydrated phases such as crystallizing phase, reflecting the higher high proportion of phenocrysts, com- hornblende and biotite. Hornblende Fe3+ /Fe2+ content of the melt (e.g. monly up to 20%. Plagioclase is by far phenocrysts are common in arc lavas, Miyashiro 1974). The enhanced stabili- the most common phenocryst in arc and both hornblende and biotite are ty of these minerals greatly affects the magmas of all compositions and typi- common in arc plutons. Experiments chemical composition of arc magmas. cally exhibits normal, oscillatory or on the cooling history of basaltic In contrast to continental rifting envi- reverse zoning (e.g. Stewart and Fowler magma formed in arc environments ronments, K-feldspar phenocrysts are 2001). At constant pressure, the com- (Foden and Green 1992) indicate that rare, even in felsic rocks, because most position of plagioclase is controlled by the stability field of amphibole relative arc magmas are not enriched in alka- the temperature, composition and to olivine, pyroxene and plagioclase, is lies. water content of the melt. If, as a pla- expanded with increasing partial pres- Water content in magmas also gioclase crystal grows, these conditions sure of water (see also Yoder and plays a key role in phenocryst develop- change, so too may the composition of Tilley 1962). The implication is that ment because it reduces the viscosity the next growth increment, leading to amphibole may, in some situations, of the melt by breaking Si-O bonds (a zoning. Zoning requires that chemical crystallize before olivine or pyroxene. process known as depolymerization). As equilibrium between the melt and the As water is an oxidizing agent, a result, ions can diffuse more easily plagioclase is not maintained. This may it also affects the stability of minerals and minerals can grow more rapidly be because the strength of Si-O and such as magnetite that contain Fe3+. In (Mysen 1988). Arc lavas of all compo- Al-O bonds inhibits the dissolution of 160

Figure 10b. Simplified NaFMKASH (Na2O-FeO-MgO-K2O-Al2O3-SiO2-H2O) modified from Brown (1993), Spear et al. (1999) showing important reactions in pelites that can generate silicic magma. Reactions shown for P = P , SiO is H2O total 2 assumed to be in excess. Note the position of each reaction in P-T space is idealized and changes as a function of a (except H2O for vapour-absent reactions) and/or Fe-Mg solid solution (where Fe-Mg bearing phases are involved). Reactions involving biotite are thought to produce magma with sufficient volume to attain the buoyancy required to rise to higher structural levels (e.g. Clarke 1992). GWS, Granite Wet Solidus (combination of reactions 2 and 5); BWS, Basalt Wet Solidus (see Brown 1993). plagioclase, which is required to main- role in all stages of arc development, RELATIONSHIP BETWEEN MAGMA- tain equilibrium with the cooling melt, starting with magma generation, then TISM, METAMORPHISM AND or because of the slow diffusion of Al. during its ascent, and finally during its DEFORMATION IN ARCS Normal zoning is thought to reflect cooling history. It also influences the Over the last 30 years, it has become cooling of the magma, oscillatory zon- stability of minerals in the source rock, clear that maximum pressures and tem- ing reflects abrupt changes in melt thereby affecting bulk partition coeffi- peratures are acquired at different composition, possibly brought on by a cients and magma chemistry. Water times during the progressive evolution new influx of magma into the cham- imparts chemical, mineralogical and of orogenic belts (e.g. Houseman et al. ber, and reverse zoning is thought to textural characteristics at all stages of 1981; Brown 1993). This conclusion represent a small component of oscilla- magma evolution. has profound implications for the gen- tory zoning (e.g. Shelley 1993). esis of arc magmas and their relation- In summary, water derived ship to metamorphism and deforma- from the subducted slab plays a crucial tion of crustal rocks. Through a com- GEOSCIENCE CANADA Volume 33 Number 4 December 2006 161

by thrust and reverse faults, particularly partial melting in the mantle and crust. in zones with rheologies that have This thermal phase is followed by been weakened by rising isotherms. crustal thickening which may be coeval The thermal effects of crustal thicken- with mantle thinning (e.g. Sandiford ing due to thrusting have been mod- and Powell 1991; Brown 1993). elled (Figs. 12, 13) by England and According to Collins (2002), however, Thompson (1986) and Thompson and as most granulite terranes record high- Connelly (1995). For a thrust sheet 35 er geothermal gradients (25-50°C km-1) Figure 11. Schematic representation of km thick, the net effect is to put the than those reported in simple crustal a Mohr stress analysis (the relationship lower half of the thickened crust under thickening models (Fig. 13), volumi- conditions of melting. Initial magma nous magmatism and high-grade meta- between U normal and U shear, normal and shear stresses) showing the generation takes place at the base of morphism may preferentially occur role of fluid pressure in creating frac- the crust and migrates upwards with during episodes of extension in the arc tures in the lithosphere. The maxi- time (Fig. 13) as successively higher during slab roll-back. However, the structural levels cross the minimum inevitable arrival of microcontinents mum principal stress is given by U1, melting curve. Note that the paths for and/or oceanic plateaux at the trench the minimum principal stress by . U3 initial post-thrusting depths of about may induce temporary compression, in Dry conditions, (thin line D), hydrated 50 km cross the minimum melting which the rheologically softened crust conditions, thick line H). In a devia- curve about 15 million years after undergoes thickening driving the P-T-t toric stress field (given by - , the U1 U3 thrusting. Should extensional exhuma- path to higher pressures (into a CCW diameter of the circle) increasing tion accompany thrusting, the time path). In this scenario, crustal melts pH2O (T) reduces the effective princi- interval between thickening and mag- become deformed as they cool below pal stresses by T, shifting the Mohr matism would be reduced in duration. near-solidus temperatures in a devia- Circle to the left and ultimately creat- The above model provides insights toric stress field (e.g. Hibbard and ing fractures when the Mohr Circle into the spatial and temporal associa- Watters 1985). intersects the Mohr (fracture) enve- tion of magmatism, metamorphism lope. When this occurs, fluid may be and deformation exhibited in many arc CONCLUSIONS lost from the system, and the circle regimes. There are two types of volcanic arcs – shifts temporarily to the right, until Counter-clockwise (CCW) P- intraoceanic island arcs that may be fluid builds up and repeats the process T-t paths are generated when heating underlain by either oceanic or conti- (Sibson 1987). (and magmatism) precedes crustal nental crust, and continental arcs that thickening. Such paths can be initiated form along the edge of a continent and bination of thermodynamic modelling, by underplating of the crust by mafic are underlain by continental crust. laboratory experiments and isotopic magma, possibly during a local or Island arcs underlain by oceanic crust and fission track dating, changes in regional extensional event (Fig. 14). are dominated by mafic magmas, pressure and temperature during the Underplating probably occurs because whereas continental arcs and island temporal evolution of orogenic belts of viscosity contrasts at the crust-man- arcs underlain by continental crust are can be documented (Pressure-Temper- tle boundary. As its ascent is arrested, dominated by intermediate to felsic ature-time paths, or P-T-t). Some arcs the mafic magma spreads out laterally magmas. Arc magmatism is closely have clockwise, others have counter- and heat is transferred to the base of related to metamorphism, which may clockwise P-T-t paths. the crust as the magma cools. If weak- follow either clockwise or counter- Clockwise (CW) P-T-t paths nesses in the crust are exploited, mafic clockwise P-T-t paths. Clockwise P-T- are generated by crustal thickening fol- magmas may rise to relatively shallow t paths are initiated by crustal thicken- lowed by erosional or tectonic exhu- levels. In both situations, significant ing, associated with thrusting, followed mation. Because thrusting is viewed to heat is transferred from the cooling by a temperature increase due to the be geologically instantaneous, maxi- mafic magma into the crust and may thermal effects of thickening. Magma mum P is reached in the crust before be enough to cause crustal melting. forms first at the base of the crust but maximum T and so magma generation These tendencies may be amplified, if over tens of millions of years forms at typically post-dates thrust-related at the same time, the crust is undergo- successively higher crustal levels. deformation, thereby initiating a CW ing extension, such as in back arc Counter-clockwise P-T-t paths are ini- path. In compressional zones in arc regions. The resulting crustal thinning tiated when heating (e.g. underplating systems, the thermal regime that gives and pressure reduction leads to a of the crust by mafic magma) precedes rise to arc magmas is greatly affected steepening of isotherms and induces crustal thickening. Clockwise P-T-t 162

added fluid may influence the mineral composition of the mantle, as well as change the composition of the fluid. The composition of the fluid at the time of melting influences the chemistry of the magma produced, and therefore, the composition of the resulting arc rocks. Water also plays an important role in the ascent and cooling of magma. High pore-water pressure can induce fracturing in the crust subjected to deviatoric stress, which allows magma and fluids to rise more rapidly. Water depolymerises melt structure, and the resulting decrease in viscosity allows minerals to grow more rapidly. Arc lavas, especially those of intermediate composition, are characterized by Figure 12. The effect of crustal thickening and thinning on the geothermal gradi- plagioclase phenocrysts with oscillatory ent (after England and Thompson 1986; see also Thompson and Connolly 1995). zoning that records variations in tem- Thickening is assumed to have taken place instantaneously by a single thrust sheet perature, composition and water con- 35 km thick (top right) and thinned to its original thickness by erosion over 100 tent of the melt during magma crystal- million years (bottom left, Model A) or by extension (bottom right, Model B). For lization. As volatiles partition into erosional and extensional models, the initial geotherm is represented by the dashed magma, source rocks may become line, the final geotherm by the solid line. dehydrated so that a < 1, which H2O paths occur when arcs are under com- sional phase, basins develop that are causes melt-generating reactions to pression and counter-clockwise paths filled with sediments and a steepened migrate to higher temperatures, and occur when an arc is under extension. geothermal gradient produces exten- thus, inhibits the formation of significant quantities of magma. If, however, Both conditions can occur during the sive magmatism in the rheologically water is continually replenished, direct- lifespan of a given arc. weakened and thinned crust. During a ly or indirectly, by ongoing dehydration Arc volcanism occurs in a belt prolonged history of subduction, the of the subducting slab, then magma about 10 km wide, which is approxi- basin fill provides a fertile chemistry generation can proceed under water- mately 110 km above the subduction for the generation of magmas during saturated conditions at lower tempera- zone. However, subduction zones can subsequent phases of compression or tures. last for hundred of millions of years extension. Several mechanisms may com- and changes in the location and dip of Water plays a fundamental bine to facilitate the rise of arc magmas the subduction zone during that time role at all stages of magma evolution. towards the surface. Large plutons result in arcs that are typically 200-300 Water lowers the solidus temperatures appear to represent the end result of km wide. The width of the fore-arc in the mantle and crust because it multiple intrusions. Buoyant magma region i.e. between the arc and the weakens mineral structures. In addi- has the tendency to form teardrop- trench, also changes with the dip of tion, rising water acts as a heat flux. shaped diapirs, particularly in the duc- the subduction zone. Fore-arc crust The composition of the fluid at the site tile lower crust. Many diapirs appear to and overlying sediments may become of melting has a major influence on the have exploited zones of structural imbricated and transferred to the sub- composition of the magmas produced. weakness in the crust, where rapid ducting slab, a process known as sub- This fluid probably has components of ascent resulted in expansion and fur- duction erosion. the invading fluid from the subduction ther fracture propagation in the roof The backarc region, another zone, the indigenous fluid previously above the magma chamber, facilitating important site of magmatism, is char- added to the mantle wedge, and the further rise by stoping and assimilation. acterized by high heat flow, and under- fluid released by mantle minerals. The presence of water in magmas also goes repeated episodes of extension Thus, reaction between the invading aids in fracture propagation. Volatile and compression. During the exten- fluid, mantle minerals and previously overpressure and extensional tectonic GEOSCIENCE CANADA Volume 33 Number 4 December 2006 163

regimes in the arc (e.g. local pull-apart structures) permit rapid rise of magma. Significant volumes of magma migrate through a variety of structural weaknesses and the net result may be a composite batholith that is typical of arc systems.

ACKNOWLEDGEMENTS Much of what I have learned about arc magmatism has been through collabo- ration and conversations with col- leagues including Alan Anderson, Randy Cormier, Richard D’Lemos, Jarda Dostal, Andrew Hynes, Duncan Keppie, Damian Nance, Georgia Pe- Piper, Cecilio Quesada, Rob Strachan and participants in various UNESCO- IGCP projects. I am grateful to NSERC Canada for research funding, for which arc systems are a major part, to Randy Cormier for reading an early draft, to Randy, Derek Thorkelson and Steve Johnston for thorough, constructive reviews, Georgia Pe-Piper, Steve McCutcheon and Sonya Dehler for editorial advice, and to Matt Middleton (funded by NSERC Research Capaci- ties Development grant) for cheerful technical assistance.

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reactions in the system KAlO2- the role of the Juan Fernández

NaAlO2- Al2O3- SiO2- H2O: Conti- Ridge in the tectonic evolution of 168 GAC NEWFOUNDLAND AND LABRADOR SECTION TO MARK THE 30TH ANNIVERSARY OF A VERY FAMOUS MAP

Following a long-standing tradition, and in keeping with a keen sense of the absurd, the 2007 Spring Technical Meet- ing of the Newfoundland and Labrador Section will once again be held in the depths of winter, at the remarkable Johnson GEO CENTRE on scenic Signal Hill in St. John’s. This will be a special meeting, for it will mark the 30th anniversary of a very famous and influential map, that laid the foundation for many of today’s thematic geological maps. Best-selling author Simon Winchester coined the phrase “The map that changed the world” for William Smith’s very first geological map of the British Isles. The Newfoundland and Labrador geoscience community has a similar label for a map that changed many of our perceptions about the geology of orogenic belts. We call it

“The Map that Moved Mountains”

The spring meeting (Feb 19-21st, 2007) will feature a symposium to mark the 30th anniversary of the ground-breaking “Tectonic-lithofacies map of the Appalachian Orogen”, first developed by Dr. Harold (Hank) Williams in 1977. This remarkable piece of work was the first to depict orogen-scale tectonic elements in terms of their original settings and locations, and its influence has pervaded all subsequent studies of the Appalachians and influenced those in other ancient orogens. The symposium will also honour Hank’s long and distinguished career as a teacher, researcher and mentor, and his huge contribution to our geological knowledge of Newfoundland, North America and western Europe. The plan for this symposium is developing steadily, and it will feature contributions related to the Appalachian-Caledonian Orogen, especially those with broad and provocative Hank Williams around the ideas, contributions related to the evolution of mountain belts in general, and also con- time he first developed the tectonic-lithofacies map of the tributions related to the evolution of geological maps and how they have influenced our Appalachians (ca. 1977). thinking. St. John’s will be welcoming several guest speakers from Canada and the Unit- Those of you who know Hank ed States. These include Jim Hibbard (North Carolina State University), Bob Hatcher would agree that he doesn’t look (University of Tennessee), Bob Tracy (Virginia Tech), Paul Hoffman (Harvard Universi- much different today! ty), Cees Van Staal (GSC, Vancouver) and Phil McCausland (University of Western Ontario). The organizers are now in the process of confirming additional guest speakers for the final program. There will also be contributions from members of the Newfoundland and Labrador geoscience community. There will also be a general session for the presentation of papers on an eclectic range of topics, as is normally the case at these meetings. Social events are planned for the evenings of Monday 19th and Tuesday 20th of February. The plan for Tuesday night will see a gathering at one of St. John’s most famous watering holes, “Bridie Molloy’s”, with live entertainment from local musicians who moonlight as amateur geologists. Or is it the other way around? There are few good things that you can say about the late February weather in St. John’s, but it is the season of low airfares in and out of North America’s oldest city. Those who make last-minute plans to attend won’t have huge expenses, and they are guaranteed a stimulating meeting and a very good time. Pre-registration continues until the day of the meeting, and it only costs $50 for professionals and $20 for students. For further information about this meeting, consult the GAC Newfoundland and Labrador Section website at [www.gac-nl.ca]. It is updated on a regular basis. The abstract deadline is technically past, but the province has its own time zone, and interesting submissions will still be considered. For enquiries not answered by the comprehen- sive website, check with the Technical Program Chair (Andrew Kerr) at [[email protected]].