Structural and Tectonic Evolution of K-Rich Silica-Deficient Volcanic Provinces of Different Continents - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 441

Smoky Butte (SB) are shown in Fig. 14.11 . During the period (55 50 Ma,

Fig. 14.11 a), relatively rapid convergence of the Farallon plate resulted in low-
angle subduction, with a trench approximately 750 km southwest of the Highwood
Mountains. Fluids enriched in LILEs released at relatively shallow depths from the
subducted slab reacted with the overlying asthenospheric mantle to form a phlog-
opite

-

and amphibole-bearing peridotite carapace that was dragged down by
induced

—

flow. The Challis Volcanics probably formed as a result of amphibole
breakdown in this metasomatised carapace.
Figure 14.11 b shows that as the slab descended to greater depth below the
Wyoming Province craton, breakdown of residual phlogopite in the peridotite
carapace induced melting to form the proto-Highwood magmas. In an alternative,
two-stage model the carapace components probably had been added to the wedge
during the late Cretaceous or Palaeocene, and reactivated in the Eocene by back-arc
asthenospheric

fluid-rich mag-
mas ascended to the lower lithosphere interface, but only those magmas (solid lines)
that migrated along pre-existing pathways (dashed lines) where they retain suffi-

flow. In either case, these small-volume LILE and

-
cient heat to rise very far into the thick lithosphere. Figure 14.11 c demonstrates that
this preferential channelling facilitated the movement of alkali magma, which
reacted with ancient crystallization and metasomatic products present as veins
within the depleted harzburgite mantle lithosphere. Assimilation of these Ba-rich
metasomes overprinted the Highwood magmas with the distinctive ancient U-Th-
HFSE-depleted Ba-LREE-enriched geochemical/isotopic signatures. The degree of
overprinting was variable depending on residence time (e.g. more for HM-622,
considerably less for HM-626b). At a slightly later time (48 Ma), CO 2 fluid or

carbonatite melt fluxing in the asthenospheric mantle included melting to from the

alnoitic suite of magmas represented by Haystack Butte and the Missouri Breaks
diatremes. These magmas ascended explosively through the lithosphere and
acquired very little of the ancient metasome signature. In Oligocene (27 Ma), very
small volume of lamproitic melts containing a dominant ancient metasome com-
ponent were generated within the deep lithosphere (either by in ux of back arc

asthenospheric melts or by unspeci ed processes linked to extension) and erupted at

Smoky Butte. According to the model, about 55 50 Ma ago, there was rapid low

angle subduction of the Farallon plate (Dickinson 1979; Bird 1984), along a trench
located approximately 750 km southwest of the Highwood Mountains (Heller et al.
1987). O

-

fluid enriched in LILEs, released at shallow
depths from the subducted slab reacted with the overlying mantle to produce
phlogopite-amphibole-bearing peridotite carapace. With the progressive descent of
the slab to a greater depth below the Wyoming Craton, breakdown of amphibole-
bearing segments resulted in the production of Chalis Volcanics. This was followed
by breakdown of phlogopite in the peridotite carapace. Partial melting of the
phlogopite-rich segment resulted in the production of magmas that intruded the
Highwood Mountains localities. They considered that magmatism in that area
would be related to a two-stage process (Fig. 14.11 b). Addition of carapace com-
ponents perhaps were added during the Cretaceous or Palaeocene and became
reactivated during Eocene due to back-arc asthenosperic

' Brien et al., thought that

flow. A very small volume