The Signature of Amphibole in Mafic Alkaline Lavas, a Study in the ...

JOURNAL OF PETROLOGY VOLUME 36 NUMBER 5 OCTOBER i 995
B
i
| Negr>
i
1
K
+
Na
cations
30
20
10
—
-
Ol-NEPH
BASAN
n^. AOB
Jad
Ag
•
f
Ortho
Albite
—
—
Hirschfeld Creek
N
A
Ol
Af
I
J
i d-
D-
40 50 60 70
cations
Fig. 2. Total alkalis vs Si plot in cations of lavas with >8 wt%
MgO in the Cordilleran alkaline volcanic suite. •, Ol-NEPH;
A, BASAN; •, AOB; •, Hy-NORM AOB.
The Mount Llangorse volcanic field in northern
British Columbia is dominated volumetrically by a
thick lava lake or tuya of AOB south of Mount
Llangorse (59°22'N, 132°47'W), which has a vesiculated
basal contact and numerous spinel lherzolite
xenoliths in its lower portion. This unit overlies a
bedded agglomerate containing numerous bombs of
Ol-NEPH and xenoliths of amphibole biotite granodiorite
and spinel lherzolite. Table Hill (59°20'N,
132°54'W) is a smaller erosional remnant of ponded
lava flows, to the south of Mount Llangorse, whose
uppermost flow is composed of Ol-NEPH. These
volcanics are associated with four small high-level
intrusions interpreted to be the erosional remnants of
volcanic conduits, all aligned along a north-south
topographic lineament. Three of these plugs are
composed entirely of Ol-NEPH with abundant spinel
lherzolite xenolith fragments, as well as megacrysts
of clinopyroxene, amphibole and rare garnet pyroxenite
xenoliths (Fire Mountain, 59°27'N,;132°47'W,
Higgins & Allen, 1985; Lone Point 59°18'N,
132°47'W; and one under the bedded agglomerate at
Mount Llangorse). Most impressive, however, is a
columnar-jointed plug, 500 m in diameter, on a
tributary of Hirschfeld Creek (59°32'N, 132°56'W),
which exhibits an outer margin of Ol-NEPH (9 m)
and an interior zoned from BASAN to AOB over a
distance of 3 m, followed by an asymptotic gradation
to a Hy-NORM basaltic core (Fig. 3). Small
irregular fragments of anhydrous spinel lherzolite are
abundant in the outer Ol-NEPH margin of the
Hirschfeld plug, but become larger and ovoid across
the contact to the BASAN interval, and then disappear
at a distance of 5 m into the AOB interior.
1
500 m
20 m contours
Fig. 3. Detailed map of the Hirschfeld intrusive plug showing the
relationship between the outer Ol-NEPH margin and the interior
AOB core. The numbered black lines mark the two sampled sections;
#, locations of two interior samples. For clarity, the width of the Ol-
NEPH border has been exaggerated; its true width is ~9 m.
The contact between the outer Ol-NEPH margin
and the interior BASAN is almost invisible in the
field because of the macroscopic similarity of the two
rock types and the fact that columnar joints pass
through it undisturbed. It is best identified in the
field by the rapid inwards increase in grain size in
the BASAN adjacent to the contact over a distance
of a few centimetres just before large ovoid spinel
lherzolite xenoliths become abundant. The contact
between Ol-NEPH and BASAN dips into the intrusion
and the highest concentration of large lherzolite
xenoliths is found in a 15-m interval overlying the
contact, where it dips gently inwards along the
southwestern margin of the intrusion, suggesting that
the xenoliths have settled from the interior of the
intrusion. Some of the larger xenoliths, however,
actually penetrate the contact into the outer Ol-
NEPH margin.
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