Geology Reference
In-Depth Information
According to Zhang et al. (1995), asthenospheric and crustal attenuation in
Songliao basin have been established by seismic and electric conductivity mea-
surements. They noted that the lithosphere at the centre is 80 km thick, 40 km
thinner than that in the margins. The rocks from this locality are porphyritic with
groundmass having vitreous to holocrystalline texture. These potassic rocks are
subdivided into olivine leucitite, leucite basanite and trachybasalts. Olivine leuci-
tites are distinguished from leucite-basanites and trachybasalts by the presence of
nepheline and paucity of feldspars and high modal contents of ma
c phases and
leucite. The basanites contain leucite but this mineral is absent in trachybasalts.
Olivine leucitites are found from three volcanoes:
(1) Dangzishan,
(2) Heishan, and
(3) Muhenanshan of the Keluo
field.
The Muhenanshan volcano is the only one known so far, which is characterized
by the outpouring of olivine leucitite and trachybasalts in different episodes of
magmatism (Fig.
4.1
).
Zhang et al. (1995) described the presence of mantle-derived spinel peridotite
xenoliths at several volcanoes of each
field. Orthopyroxene, clinopyroxene and
sanidine megacrysts and various igneous inclusions are found from Dangzishan,
Heishan and Gushan volcanoes (Keluo).
The WEK volcanic rocks are characterized by high K
2
O (3.5
7.1 wt%), K
2
O/
-
87
Sr/
86
Sr
Na
2
O ratio (>1)
and incompatible
elements. They have high
143
Nd/
144
Nd (0.51238
206
Pb/
204
Pb
(0.7050
0.7056),
low
0.51250) and low
-
-
(17.06
16.61) ratios. The major element analyses of some of the WEK rocks are
given in Table
4.1
.
Zhang et al. (2002) studied the Holocene volcanoes in the Jingpohu region,
which are situated in the
-
areas along the northwest
side of Jingpohu Lake, northeastern China. They dated three charcoal samples from
the
“
Crater Forest
”
and
“
Frog Pool
”
first and second volcanic cycles, and found that the ages of the
rst and second
cycles are 3,430
3,490 and 2,470 years, respectively. The lavas from the Jingpohu
area consist of basanites (BSN), alkali olivine basalts (AOB) and tephrites (TP).
According to them Crystal fractionation models are consistent with the generation
of AOB and TP from a basanitic parent. Minor fractionation of olivine, augite,
magnetite and Cr-spinel is required to produce AOB compositions whereas the
generation of TP requires extensive fractionation of kaersutite, phlogopite and
anorthoclase with minor olivine, augite, magnetite and leucite. They suggested that
the presence of kaersutite, phlogopite and anorthoclase megacrysts and mantle
xenoliths were, part of a fractionation history at high pressure in the mantle.
Although all basaltic rocks contain many granitic xenoliths, their geochemical
characteristics show that they have not undergone any contamination of upper crust
en route to the surface, but some alkali basalts might have suffered from contam-
ination with lower crust. Relatively unradiogenic isotope ratio compared with Bulk
Earth, steep chondrite-normalized REE patterns and strong incompatible element
enriched patterns suggest that the magmas were derived from a mixture of an
-
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