149 / 352
133
7.2.1. Sample GCH-25
(Sismundi leucosome)
This sample was collected from
Sismundi
(43°42’25.52”N, 7°52’14.66”W; Simple Cylindri-
cal projection, WGS84 datum; Appx. 1).
This sample is interpreted from field
observations as a leucosome, formed by partial
melting of the metasedimentary rocks (Fig. 1).
This lithology is displayed as centimetre sized
leucocratic bands, folded and coherent with the
metasedimentary rocks regional foliation (Ref:
114088; Ref: rock collection reference, UCM).
From microscopic observations this sample
has Qz and Afs as major constituents (Fig. 2).
Both minerals are poikilitic and feldspar is
usually altered and shows pertitic exsolutions.
Qz contains minor inclusions of Bt, Ap, Zrn and
Spn, and Afs contains Grt, Qz, Bt, Ap and Zrn.
Minor constituents are Grt, Ap, Bt, Chl, Amp
(scarce), Spn and Zrn. Grt is small, idioblastic,
has a small amount of inclusions (probably Rt, Ap
and Zrn) and it is not surrounded by Bt or other
minerals (except feldspar). These observations
clearly suggest that Grt grew in this leucosome
instead of being a relict mineral mechanically
extracted from the metasedimentary rock
(mineral abbreviations after Whitney & Evans,
2010).
7.2.1.1. Results
From sample GCH-25, 54 U–Pb analyses
were performed of which 35 were concordant
(
d
= 35.2%;
d
, percentage of discordant analyses;
Figs. 3 & 4a,b). Domain II is defined by 26
Devonian analyses constrained between 377
and 403 Ma, and show a well-defined maximum
abundance at 387 Ma (all zircon analyses from
this population are concordant). Domain I is
defined by 7 Cambrian analyses, that show badly
defined relative abundance peaks at 496, 490 and
480 Ma (only 7 out of 21 zircon grains from this
population were concordant). One analysis is
Paleoproterozoic and one is Archean (considered
inheritances; only two out of seven zircon grains
from this inherited population were concordant;
Fig. 4b).
Most of the domain I zircon (Cambrian) show
more or lesswell definedoscillatory zonings.They
are normally surrounded by lobulated sinuous
rims, which are dark in CL images and bright
in BSE images. Domain II (Devonian) zircon is
displayed as rims surrounding domain I zircon.
They show well defined combined oscillatory
and multi-faceted (“soccerball”) textures. These
rims are the outer features observed and define
the crystals exterior habit, which shows well
defined crystal faces and the long crystal axis to
be between two and five times longer than the
Fig. 1.
Field photograph of the studied sample. The hammer
is 33 cm long.
Fig. 2.
Photomicrograph of the studied sample.
ȝP
7.2. LEUCOSOMES