Structural and Tectonic Evolution of K-Rich Silica-Deficient Volcanic Provinces of Different Continents - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 440

42 Ma) also cover central Idaho, southwest
Montana and further west in Oregon and Washington. Close association of calc-
alkalic and K-rich silica-undersaturated rocks in Sunda arc and Mexico, provided
indirect support for the model that suggested that the Eocene volcanism of the
northwestern United States to subduction-related process. O

Voluminous calc-alkalic rocks (46

-

Brien et al. considered
that the recent minette lava in western Mexico was erupted in an extensional
environment, and the volcanism was similar to the eruption of such rocks in the
northwestern United States during the Eocene (Coney 1980). Lipman (1983) and
Heller et al. (1987) reconstructed the tectonic history of the region for this period.
According to their hypothesis,

'

SE-trending convergent plate
boundary throughout the western United States. Lipman et al. (1971) and Dickinson
(1979) thought that there was a single low dipping slab of Farallon plate subducting
under the lithosphere in this region.
O

there was a NW

-

Brien et al. (1991) pointed out the presence of calc-alkalic rocks in the
southwest within the Larmide fold and thrust orogenic belt to potassic rocks within
the foreland, is consistent with the deeper and mineralogically distinct source region
below the Wyoming cratonic knell. They therefore, considered that the genesis of
parent magma for the Highwood Mountains region is related to partial melting of
asthenospheric mantle wedge, which resulted in the in

'

fluids from a low-
angle subducted slab (Fig. 14.11 ). The relationship between the subduction slab
with respect to the position of different volcanic fields at Challis, Rocky Mountains,
Highwood Mountain (HM), Haystack Butte (HB), Bearpaw Mountains (BM) and

ltration of

WSW ENE

Challis Rocky

Mtns.

HM BM

SB ( 27 Ma )

HB

(a)

Archean

Proterozoic

(c)

(b) Depleted

Harzburgite

BM

HM HM HB

626 622

Ba LREE - Rich LILE Fluid Rich

Magma

U Th-Poor
Metasome

Fig. 14.11 Schematic cross section illustrating tectonic model for the formation of the Highwood
Mountains magma (no vertical exaggeration) (after O ' Brien et al. 1991)