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Mineralogical and geochemical constraints on contribution of magma mixing and fractional crystallization to high-Mg adakite-like diorites in eastern Dabie orogen, East China

State Key Laboratory of Geological Processes and Mineral Resour ces, Faculty of Earth Sciences, China University of Geoscienc es, 430074 Wuhan, China

Institut für Mineralogie, Leibniz Universitt Hannover, Callinstr. 3, D-30167 Hannover, Germany

Instit ut für Mineralogie, Westflische Wilhelms-Univers it

t Münster, Corrensstr. 24, D-48149 Münster, Germany

abstractarticle i nfo

Article histo ry:

Received 16 January 2013 Accepted 13 April 2013 Available online 20 April 2013

Keywords:

Magma mixing Fractional crystallizatio n High-Mg adakite Dabie orogen Tan-Lu fault MLYR belt

The Liu jiawa p lu to n which is l ocate d n e ar the e ast ern b ou ndary o f t he Dabie orogen is c om po sed of mul ti ple lith- ologic units including mainly gabbronor ites, d iorites, granod iori tes and hornblende gabbros. Gabbronori te s and hornble nde gabbros occur as enclave s in di oritic hosts wh ich show gradu al contact w ith granodiorites. Zi rcon U–Pb dati ng i ndicate s t hat gabbronori tes and diori tes fo rmed co evall y at ~ 128 Ma, but they have distinct zircon Hf isotope s wi th εHf (t) o f −26 to −23 (gabbronorite ) and of −32 to −27 (diorite) respectively. Petrographic o b se rv at io ns and r ock-fo rming mine ral co mpo s it ions cl ea rly sho w mi xing b e tw een ma fic a nd f elsic magma end-members, which might have formed the homogeneous whole-rock Sr–Nd isot opes with εNd ( t ) o f −17 to

Sr of 0. 70 7 to 0 . 709. As re vealed by zircon Hf isotope s, F concentrations in am phibole and biotite and thermodynamic modeling of crystallization, the gabbro norites r epresent enriche d l ithospheric mantle-derived magmas which evolved by fractional crystalli za ti on of o r th op y r ox en e, clino pyro xene, m agne tit e and/or amphibol e, whe re as the granodiori tes may be derived f ro m the Dabie Archean basem ent. M ineralogical and ge ochem ic al data as wel l as m ajo r and trace e lem ent m odeli ng show that the o rigin o f di orites, previously interpre ted as h i gh-Mg adaki te s, can be ex plai ned b y magm a mixi ng be tw een th e crust -deri ve d granod ioriti c m agmas and t he dif ferentiation p roducts o f mantle-derived gabbronor itic magm as. As a resul t, t he high-Mg ad akit e-lik e g eo chemi st r y of the di or it es is a co n sequ ence o f magm a d iffe re ntiat io n at a c ru stal de pt h, invo lving fracti on al crys talli za ti on and magm a m ixing, r athe r t ha n a nintrinsicfeatureofprimitive melts.Themantleupwell- ing in the adjacent central Middle-Lower Yangtze River metallogenic (MLYR) belt during Late Jurassic–Ear ly Cretaceous belt m ight h ave acted as a precurs or and tri gge red the p artial me lt ing o f litho sphe ri c mantle b eneath the e astern Dabi e oroge n and the f urthe r me lt ing o f oroge nic basem ent, consistent wi th the mode l of Zhang et al. (2010) suggesti ng a magmatic li nk betwee n t he ML YR bel t and the so utheaste rn D abi e o rogen. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

In arc environments, adakitic andesites have been frequently interpreted as products of direct partial melting of subducted oceanic crust (Defant and Drummond, 1990; Kay, 1978; Martin et al., 2005; Yogodzinski et al., 2001). The adakitic signatures (e.g. high Sr but low Y concentrations) can be formed by high-pressure melting of basaltic rocks leaving garnet as a stable phase. Interaction between melt and mantle wedge (peridotite) can substantially promote the Mg content of ascending melts ( Kelemen, 1995; Liu et al., 2012; Tsuchiya et al., 2005; Wood and Turner, 2009). Comparatively, in intra-continental environments where oceanic subduction is absent, high-Mg adakitic intrusive or volcanic rocks have been widely be- lieved to represent partial melts of delaminated lowermost part of thickened continental crusts at great depths ( Gao et al., 2004; Guo et al., 2006; Huang et al., 2008; Qin et al., 2007; Xu et al., 2002, 2006, W. Xu et al., 2008b; Zhang et al., 2010 ). However, if the high-Mg adakitic magmas observed in arc or intra-continental envi- ronments are not primitive, as evidenced by numerous studies using mineralogical and geochemical ( Barboni et al., 2011; Castillo et al., 1999; Chiaradia et al., 2009; Dessimoz et al., 2012; Gao et al., 2009; Guo et al., 2007; Li et al., 2009; Macpherson et al., 2006; Richards and Kerrich, 2007; Streck et al., 2007; Zhang et al., 2011b) as well as experimental studies (e.g., Alonso-Perez et al., 2009; Müntener and Ulmer, 2006), their high-Mg adakitic signatures might be related to other processes including magma mixing and/or fractional crystallization of a variety of minerals (e.g., pyroxene, garnet and amphibole). These rocks and corresponding magmas can be termed as “adakite-like ”or “pseudo-adakite”and their geological implications are thus distinct from those based on assumptions that they are primitive magmas (e.g., Chiaradia et al., 2009; Li et al., 2009). For cases where magma mixing and/or crystal accumulation are evident from field and microscopic observations, it is critical to identify the involved end-members and their origins using multiple approaches prior to interpret their pristine geochemical signatures properly. In th e T an -L u f ault zone of East Ch in a, wh ic h is a dj ac en t t o the ea st- er n b ou ndary of the Dabi e oroge n, several L ate Mesozoic m ag am tic in trusions (inclu ding th e C hilu ti ng diorit e) have been sugge st ed to be high-M g a da ki tes resul ting fr om partial m elting of dela minated c on ti- nental crust a nd subsequent melt/mantl e i nteraction (Huang et a l. , 2008; Wang e t a l., 2006, 2004 a). However, up to now d etailed petro- gr aphical a nd mineralogical i nves tiga tion s t o c heck wh et her t hey rep - re sent pr im it iv e m agmas o r n ot are l ac king. F or exampl e, based on t he bulk geochemist ry of se le cted samples, th e Chilu ti ng dior it es (i.e. p art of the L iujiawa plutonic c om plex) h av e b een i nterpreted as primitive high-M g a da kites a nd a lit hospheric d elamin at ion m od el alon g t he Tan-Lu fault z on e h as be en prop os ed accord in gly (Huang et a l. , 2 008). Neve rt heless, o ur prelim in ary petrogr aphical o bs erva tion and geo- chemic al mode ling in dicate that the d io ritic m em be rs of th e L iu jiawa pluton mi ght have b een a ffected t o a l a rge extent b y mag ma mixing and f racti onal c rystal lizati on ( Zhang e t al., 2012a, 2 011b). In this paper, quantit a ti ve constr aints on t he cont ribut i on of mag ma m ixing and f rac- ti onal c rystall izati on p roce sses t o t h e high- Mg adakiti c ge ochemist ry of th e L i ujiawa pluto n a re prov ide d. W e c om bi ne pet rog raphic o bser va- ti ons, dat a o n who l e- rock geoch emistry ( major and t race e l ement , Sr –Nd isotopes), U–Pb and Hf i sot opes of z i rcon, major a nd t race ele ment compositi ons o f mine rals (pyroxe ne, amphibole , biot ite , apat it e a nd pla- gi oclase) a s well a s g eoche m ical modeli ng t o wor k o ut the pe tr ogen es is of the p lut on. We demo nst rate that the diori tic r ocks fro m the Liujiawa plut on are “pseudo high-M g adakites”and that the magmas ne it her deri ved di rect l y b y hig h-pressur e melt ing o f lo wer c on tine ntal cr ust nor reflect c rust de laminati on.

be termed as adakite-like or pseudo-adakiteand their geological implications are thus distinct from those based on assumptions that they are primitive magmas (e.g., Chiaradia et al., 2009; Li et al., 2009). For cases where magma mixing and/or crystal accumulation are evident from eld and microscopic observations, it is critical to identify the involved end-members and their origins using multiple approaches prior to interpret their pristine geochemical signatures properly. In th e T an -L u f ault zone of East Ch in a, wh ic h is a dj ac en t t o the ea st- er n b ou ndary of the Dabi e oroge n, several L ate Mesozoic m ag am tic in trusions (inclu ding th e C hilu ti ng diorit e) have been sugge st ed to be high-M g a da ki tes resul ting fr om partial m elting of dela minated c on ti- nental crust a nd subsequent melt/mantl e i nteraction (Huang et a l. , 2008; Wang e t a l., 2006, 2004 a). However, up to now d etailed petro- gr aphical a nd mineralogical i nves tiga tion s t o c heck wh et her t hey rep - re sent pr im it iv e m agmas o r n ot are l ac king. F or exampl e, based on t he bulk geochemist ry of se le cted samples, th e Chilu ti ng dior it es (i.e. p art of the L iujiawa plutonic c om plex) h av e b een i nterpreted as primitive high-M g a da kites a nd a lit hospheric d elamin at ion m od el alon g t he Tan-Lu fault z on e h as be en prop os ed accord in gly (Huang et a l. , 2 008). Neve rt heless, o ur prelim in ary petrogr aphical o bs erva tion and geo- chemic al mode ling in dicate that the d io ritic m em be rs of th e L iu jiawa pluton mi ght have b een a ffected t o a l a rge extent b y mag ma mixing and f racti onal c rystal lizati on ( Zhang e t al., 2012a, 2 011b). In this paper, quantit a ti ve constr aints on t he cont ribut i on of mag ma m ixing and f rac- ti onal c rystall izati on p roce sses t o t h e high- Mg adakiti c ge ochemist ry of th e L i ujiawa pluto n a re prov ide d. W e c om bi ne pet rog raphic o bser va- ti ons, dat a o n who l e- rock geoch emistry ( major and t race e l ement , Sr Nd isotopes), UPb and Hf i sot opes of z i rcon, major a nd t race ele ment compositi ons o f mine rals (pyroxe ne, amphibole , biot ite , apat it e a nd pla- gi oclase) a s well a s g eoche m ical modeli ng t o wor k o ut the pe tr ogen es is of the p lut on. We demo nst rate that the diori tic r ocks fro m the Liujiawa plut on are pseudo high-M g adakitesand that the magmas ne it her deri ved di rect l y b y hig h-pressur e melt ing o f lo wer c on tine ntal cr ust nor reect c rust de laminati on.



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