Granitic rocks are a major component of the continental crust and the
many and complex problems of their origin that have confronted
geologists for over 200 years still are presenting challenges today.
Current ideas of granite formation involve lower crustal melting,
segregation, ascent (as dykes or diapirs) and emplacement in the upper
crust.
In this book we suggest an alternative model for the origin of granite
in terms of in-situ melting-intracrustal convection that physically
determines the process from partial melting of mid-upper crustal rocks
to formation of a convecting magma layer. We illustrate the model using
geological, geochemical and geophysical studies from Australia, North
and South America, Europe and China, and conclude that heat convection
within a crustal partial melt layer is essential for the formation of
granite magma and that without convection, partial melting of rocks
produces migmatites rather than granites. Granite is layer-like within
the crust, and shape and size of granite bodies reflect the geometric
relationship between an irregular upper surface of the crystallised
magma layer and depth of erosion. Repeated melting of the crust
generates downward-younging granite sequences. Chemical and isotopic
compositions of granites indicate differentiation within the magma
rather than different deep sources.
Of a number of proposed heat sources that can cause mid-upper crustal
anatexis, large-scale crustal melting and formation of a granite magma
layer is considered to be primarily related to plate convergence. A
dynamic model with examples from the western Pacific continental margin
in SE China and Tethys-Tibet is proposed to explain the relationship
between plate convergence, granite and compressive deformation of the
continental crust. Mineralisation related to granite formation,
fault-block basins, formation of continental red beds and volcanism with
examples from SE China, are also discussed in terms of the new model. In
a final section, we introduce a new rock cycling model of the
continental crust and the concept of Geochemical Fields of Elements,
illustrating the unity between the microcosm and macrocosm of the
natural world.
Audience: This book will be of interest to scientists, researchers and
students in geology, geophysics, geochemistry and economic geology
