Geology Reference
In-Depth Information
Gibson et al. therefore, thought that the cause of LILE enrichment of the Cenozoic
volcanic rocks in the western USA is thus, quite controversial. The Cretaceous to
Pleistocene magmas from the Wyoming Province, are considered to have been
derived from a very ancient LREE-rich source (Dudas 1991; Eggler et al. 1988).
They considered that genesis of the compositionally similar K-rich ma
c magmas
throughout the Cenozoic era in the western USA (Leucite Hills, Crazy Mountains)
was independent of prevailing tectonic setting. This view is thus, quite contrary to
that suggested by O
Brien et al. (1991).
There is a rigid conductive lid overlying the convecting interior (Thompson et al.
1989). They recognized a mechanical boundary layer constituting the lithosphere
immediately below the continental crust. This crustal segment is considered to have
remained isolated for geologically long period from the underlaying mantle, which
had been convecting below. Heat passes through the crustal segment by the process
of conduction and there is inherent buoyancy resulting from earlier chemical dif-
ferentiation. The mechanical boundary layer below the continent is separated from
the highly convecting asthenosphere by a thermal boundary layer, which transmits
heat upward by convection at its base and conduction at the top. Thompson et al.
considered that as the thermal boundary layer overturned during convection on a
geological time scale, it remains chemically inherent part of the underlying
asthenosphere. If there is change in composition after a shift in tectonic regime,
there should be a period of several million years, when the overlying thermal
boundary layer retains the previous asthenospheric composition. McKenzie (1989)
proposed a petrogenetic model, according to which, there is continuous or semi-
continuous leakage of extremely small melt fractions from the convective mantle
into the overlying ridge in the subcontinental lithosphere. McKenzie (1989) pro-
posed that the continental thermal structure is usually stable for a long period, the
small batches of volatile-rich ultrapotassic liquid gets frozen within small depths.
Subsequent heating or stretching related to decompression of this region, results in
remelting of the accumulated frozen ultrapotassic liquid and their associated
metasomatic materials.
'
Search WWH ::
Custom Search