The structural and geochemical evolution of the continental crust: Support for the oceanic plateau model of continental growth

T h e problem of the origin of the cont inental crust can be resolved into two fundamenta l quest ions: (1) the lo­ cation and mechani sms of init ial m a n t l e extract ion of the primitive crust and (2) the processes by which this primitive crust is converted into the cont inental crust that presently exists . W e know that Archean cont inen­ tal crust is compos i t iona l ly dist inct from younger con­ tinental crust. Archean m a g m a t i s m was dominant ly bimodal , mafic thoele i i t ic plus dacit ic , heavy rare earth e lement depleted, in contrast t o the dominant ly unimodal , roughly andesit ic calc-alkaline m a g m a t i s m on younger crust [Taylor and McLennan, 1985; Condie, 1989]. T h e problem is whether these compos i t iona l dif­ ferences are primarily due t o different m e c h a n i s m s of crustal extract ion from the mant l e or t o different mech­ anisms of differentiation and al terat ion of newly formed continental crust. Most recent mode l s for the format ion of new conti­ nental crust pos tu la te tha t the crust is first ex tracted from the mant l e wi th in the ocean basins , either at is land arcs, or at oceanic h o t s p o t s (e .g . , from m a n t l e p lumes) (Table 1). T h i s new cont inental crust is then t e c h n i ­ cally accreted t o the edges of pre-exist ing cont inents by emplacement a long act ive subduct ion zones . T h u s , the majority of newly accreted cont inental crustal blocks are rapidly exposed t o subduct ion zone m a g m a t i s m , which chemical ly modifies the newly emplaced crust. Because this two-s tage process of crustal formation oc­ curs so rapidly, it is very difficult t o determine tha t fea­ tures of the cont inental crust are primary features tha t originated wi th in the ocean bas in at the t i m e of ex­ traction from the mant le , and which are secondary fea­ tures that originated at cont inental subduct ion zones . In this paper, we explore s o m e geochemica l and geo­ physical da ta sets which can provide information on the primary origin and secondary modif icat ion of the continental crust.