Geology Reference
In-Depth Information
ned Benioff zone occurring west of Sunda Strait (Sumatra).
It dips to the northeast, and has not been observed at depths greater than 250 km.
Earthquakes at depths between 500 and 650 km are relatively common towards east
of the strait. Although there is a considerable gap in seismic activity at depths
between 300 and 500 km. The deep focus events de
There is a poorly de
ne a Benioff zone dipping at
60
, although it is less steep at depths of less than 100 km.
The oldest rocks in Java and Bali are composed of lower Tertiary shallow marine
sediments which were intruded and overlain by plutonic and related volcanic rocks.
They are observed in a zone only slightly south of the present-day volcanic arc.
According to Van Bemmelen (1949), the igneous activity represents
°
first cycle of
andesite volcanism, which are relatively common in the Southern Mountains of
Java and in the Barisan Range of Sumatra. The andesite volcanism culminated in a
period of uplift and intrusion in the mid-Miocene.
A second cycle of volcanism marked by the appearance of more alkaline lavas
took place during late Tertiary. This cycle is represented by the high-K calc-alkaline
suite, which represents the second cycle, and is located further northeast from the
Miocene
'
Old Andesites
'
. These rocks are found together with the Quaternary
'
third
'
cycle of volcanism
. The distinction between the products of these latter cycles is
rarely clear.
According to Fyfe and McBirney (1975), phlogopite having higher P-T
T stability
field than other commonly-occurring hydrous phases, should survive up to a greater
depth during the underthrusting of the crust into the mantle and eventually at
considerable depths, dehydration of phlogopite should produce potassic magmas
(also see Beswick 1976). Sodium-rich amphibole, having a lower P-T
-
eld
than phlogopite, should break down at shallower depths and the magmas generated
at such depths should thus be relatively richer in sodium than potassium.
T stability
-
14.5 Plate Tectonic Model for Potassic Volcanism
in the USA
14.5.1 Tectonic History of Potassic Volcanism in
the Highwood Mountains Region
The plate tectonic model for potassic magmatism in the Highwood Mountains
region was discussed by O
Brien et al. (1991). According to them, the Montana
alkalic province in general and the K-rich ma
'
c volcanic rocks in particular con-
stitute a broad zone of Eocene magmatism, which stretches form the southern
British Columbia and part of South Dakota. According to them, the calc-alkalic
volcanism, which was contemporaneous with the Highwood potassic volcanism
(54
50 Ma) is observed in the Castle Mountain of Central Montana, the Lowland
Creek-Pioneer volcanics of the Southwest Montana and the Gallatin-Absaroka
volcanics of southern Montana and northwest Wyoming.
-
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