The Genesis of Intermediate and Silicic Magmas in Deep Crust

The Genesis of Intermediate and Silicic

Magmas in Deep Crustal Hot Zones

C.ANNEN 1*,J.

D.BLUNDY 2AND R.S.J.SPARKS 2

1SECTION DES SCIENCES DE LA TERRE,UNIVERSITE ′DE GENE `VE,13RUE DES MARAI ?CHERS,1205GENE `VE,SWITZERLAND

2DEPARTMENT OF EARTH SCIENCES,UNIVERSITY OF BRISTOL,WILLS MEMORIAL BUILDING,BRISTOL BS81RJ,UK

RECEIVED APRIL 14,2005;ACCEPTED OCTOBER 17,2005

ADVANCE ACCESS PUBLICATION DECEMBER 7,2005A model for the generation of intermediate and silicic igneous rocks is presented,based on experimental data and numerical modelling.The model is directed at subduction-related magmatism,but has general applicability to magmas generated in other plate tectonic

settings,including continental rift zones.In the model mantle-

derived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone.Numerical modelling of the hot zone shows that melts are generated from two distinct

sources;partial crystallization of basalt sills to produce residual H 2O-rich melts;and partial melting of pre-existing crustal rocks.

Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm,the magma input rate and the emplacement depth.After this incubation period,the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded.Heat and H 2O transfer from the

crystallizing basalt promote partial melting of the surrounding crust,which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions.Mixing of residual and crustal partial melts leads to persity in isotope and trace element chemistry.Hot zone melts are H 2O-rich.Consequently,they have

low viscosity and density,and can readily detach from their source and ascend rapidly.In the case of adiabatic ascent the magma attains a super-liquidus state,because of the relative slopes of the adiabat and the liquidus.This leads to resorption of any entrained crystals or country rock xenoliths.Crystallization begins only when the ascending magma intersects its H 2O-saturated liquidus at

shallow depths.Decompression and degassing are the driving forces behind crystallization,which takes place at shallow depth on timescales of decades or less.Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons.It is proposed that chemical persity in arc magmas is largely acquired in the lower crust,whereas textural persity is related to shallow-level crystallization.

KEY WORDS:magma genesis;deep hot zone;residual melt;partial melt;adiabatic ascent

INTRODUCTION

A key question in igneous petrology concerns the origin of intermediate to silicic magmatic rocks,such as volu-minous Cordilleran granite batholiths (diorites,tonalites,granodiorites and granites)and the evolved volcanic rocks (andesites,dacites and rhyolites)of destructive plate margins.The continental crust has an estimated

silicic andesite to dacite composition,with a vertical stratification from mafic lower crust to more evolved granite-dominated upper crust (Rudnick &Fountain,1995).The origin of intermediate to silicic igneous rocks is,therefore,central to understanding the evolution of the

continental crust.

In subduction settings melt is generated by partial melting in the mantle wedge where primary mafic mag-mas form by some combination of addition of H 2O-rich fluids or melts released from the subducted slab (e.g.Davies &Stevenson,1992;Tatsumi &Eggins,1995;

Schmidt &Poli,1998;Ulmer,2001;Grove et al .,2002;Forneris &Holloway,2003)and mantle decompression resulting from subduction-induced corner flow (e.g.Sisson &Bronto,1998;Elkins-Tanton et al .,2001;Hasegawa &Nakajima,2004).Experimental studies of mantle melting

(e.g.Ulmer,2001;Parman &Grove,2004;Wood,2004),

*Corresponding author.Telephone:tt41223796623.Fax:tt41223793210.E-mail:Catherine.Annen@terre.unige.ch óThe Author 2005.Published by Oxford University Press.All

rights reserved.For Permissions,please e-mail:journals.permissions@0025710d561252d381eb6e31 JOURNAL OF PETROLOGY VOLUME 47NUMBER 3PAGES 505–5392006doi:10.1093/petrology/egi084 at Institute of Geology and Geophysics, CAS on March 5, 20130025710d561252d381eb6e31/Downloaded from

and observations of the petrology and geochemistry of mafic arc magmas,indicate that primary,mantle-derived magmas range in composition from basalts to magnesian andesites (Tatsumi,1982;Tatsumi &Eggins,1995;Bacon et al .,1997;Conrey et al .,1997;Carmichael,2002,2004;Grove et al .,2002).In terms of liquidus temperatures and dissolved H 2O contents there is a range from dry and hot magmas to wet and cool varieties,even within a single volcanic arc (e.g.Sisson &Layne,1993;Baker et al .,1994;Elkins-Tanton et al .,2001;Pichavant et al .,2002a ).Volcanic rocks with MgO-rich compositions that could be in equilibrium with the man-tle wedge are rare in continental arcs and only a minor component of island arcs,an observation attributable to density filtering and intracrustal ‘processing’of ascending magmas.This processing accounts for the predominance of evolved (silica-rich)volcanic rocks and granitic plu-tonic rocks in continental and mature island arcs.The generation of intermediate and silicic arc magmas is widely attributed to two main processes:differentiation of primary magmas by crystallization within the crust or uppermost mantle (e.g.Gill,1981;Grove &Kinzler,1986;Musselwhite et al .,1989;Rogers &Hawkesworth,1989;Mu ¨ntener et al .,2001;Grove et al .,2002,2003)and partial melting of older crustal rocks (e.g.Smith &Leeman,1987;Atherton &Petford,1993;Tepper et al .,1993;Rapp &Watson,1995;Petford &Atherton,1996;Chappell &White,2001;Izebekov et al .,2004).These processes can occur simultaneously with the heat and volatiles (principally H 2O)liberated from the primary magmas triggering crustal melting (Petford &Gallagher,2001;Annen &Sparks,2002).Additionally crustal rocks can be assimilated into mantle-derived magmas (DePaolo,1981).The assimilated components may be much older than,and petrogenetically unrelated to,the arc magmas and possess distinctive trace element and isotope geochemistry.Partial melting can also occur in igneous rocks,including cumulates,that have formed from earlier arc magmas;in this case the assimilated components and arc magmas may have strong geo-chemical affinities (e.g.Heath et al .,1998;Dungan &Davidson,2004).Evidence for crustal assimilation and mixing of melts and crystals from different sources is common (Grove et al .,1988,1997;Musselwhite et al .,1989;De Paolo et al .,1992).These processes are central to models of assimilation and fractional crystalliza-tion (AFC;DePaolo,1981)and mixing,assimilation,storage and hybridization (MASH;Hildreth &Moorbath,1988).A key question is at what depth chemical differentiation occurs.Although the existence of shallow sub-volcanic magma chambers is indisputable,based on geophysical evidence as well as petrological and geological observa-tions,it is less clear that such chambers are the place where most chemical differentiation takes place.To produce igneous rocks that contain more than 60wt %SiO 2by fractional crystallization,60%or more crystal-lization of a typical primitive arc basalt is required (e.g.Foden &Green,1992;Mu ¨ntener et al .,2001).The vol-ume of parental mafic magma that crystallizes is,there-fore,typically twice as much as the evolved magma produced.As large granitoid batholiths and voluminous eruptions involve hundreds to thousands of km 3of silicic magma (e.g.Smith,1979;Crisp,1984;Bachmann et al .,2002),huge volumes of associated mafic cumulates are required.However,geological and geophysical evidence for the requisite large volumes of complementary dense mafic cumulates in the shallow crust is generally lacking.One resolution to this problem is density-driven sink-ing of mafic cumulate bodies into the lower crust (Glazner,1994).Alternatively,if differentiation of basalt occurs at deep levels in the crust then the complementary dense mafic cumulates will be located in the lower crust (e.g.Debari &Coleman,1989;Mu ¨ntener et al .,2001)where they may eventually delaminate into the mantle below (Kay &Kay,1993;Jull &Keleman,2001)thereby progressively driving the bulk crust towards andesite composition.The silica-rich residual melts generated by deep-seated basalt differentiation can be extracted and ascend,either to erupt immediately or to stall to form shallow magma chambers.If unerupted,such shallow chambers consolidate to form granite plutons,with mafic igneous rocks being a minor component or absent.Recent numerical simulations of heat transfer (Annen &Sparks,2002)and high-temperature experiments (Mu ¨ntener et al .,2001;Prouteau &Scaillet,2003)suggest a model whereby silica-rich magmas can be generated by incomplete crystallization of hydrous basalt at upper mantle and/or lower crustal depths.These observations motivate our development of a model in which basalt emplacement into the lower crust leads to generation of intermediate and silicic melts (Fig.1).Our model builds upon the concept of underplating (Raia &Spera,1997),expands on models of differentiation of basalt at high pressure (Gill,1981;Grove et al .,2002)and incorporates aspects of AFC (DePaolo,1981)and MASH (Hildreth &Moorbath,1988).We develop a quantitative model in which evolved melts are generated from H 2O-rich par-ental basalts both by partial crystallization of the basalts themselves and by partial melting of surrounding crustal rocks through heat and H 2O transfer from the cooling basalts.A key feature of our model is that melt composi-tions are determined by the depth of emplacement of inpidual basalt intrusions and thermal equilibration with the local geotherm.We refer to the site of basalt injection and melt generation in the lower crust as a deep crustal hot zone.Previous models of underplating (e.g.Huppert &Sparks,1988;Bergantz,1989;Raia &Spera,1997;Petford &Gallagher 2001;Jackson et al .,2003)have concentrated almost exclusively on melt generated

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