210 / 352
194
show a correlation when plotted against U–Pb
ages are the
H
Hf
(t)
values and
176
Hf/
177
Hf
(t)
ratios
(Fig. 90). Between
c.
520 and 500 Ma these ratios
decrease when U–Pb ages decrease, and between
c.
500 and 470 Ma the ratios increase when U–
Pb ages decrease. The
176
Hf/
177
Hf
(t)
ratio decrease
between
c.
520 and 500 Ma looks like it can only
be explained by continuous zircon crystallisation
in a liquid phase that was progressively depleted
in its radiogenic
176
Hf content. This progressive
depletion can be explained or by a
c.
20 Ma
progressive mixing between this liquid phase and
another one with lower
176
Hf/
177
Hf
(t)
ratios, or by
a continuous contamination of the liquid during
c.
20 Ma probably due to continuous assimilation
of the host-rock. The
176
Hf/
177
Hf
(t)
ratio increase
between
c.
500 and 470 Ma can be explained as,
once the rock system crystallised at
c
. 500 Ma it
started a
c.
30 Ma long partial melting process
that progressively enriched the partial melt with
radiogenic
176
Hf, that came from the
176
Lu decay
from the minerals that were melting to form the
partial melt. It is noteworthy tomention that from
field observations this orthogneiss was intruded
by the
Punta do Serrón
mafic rock (Fig. 85). This
outcrop is cartographically related with the
Punta
da Moura
mingling suite and the Eclogite Band
formation (Appx. 1). This mingling complex
is formed by a
c.
508 Ma mafic rock (sample
GCH-23; Zrn crystallisation from
c.
515 Ma;
section 7.3.4) that underwent a
c.
489 Ma solid
state recrystallisation process (that lasted from
c.
490 to 470 Ma), triggered by the temperature
input related with the intrusion of a
c.
500 Ma
felsic rock (sample GCH-14; section 7.4.4).
From these observations and interpretations it
is proposed that this orthogneiss (GCH-31) was
intruded by the mafic protolith magmas that
constitute the common eclogites of the Eclogite
Band formation (represented by sample GCH-
19; section 7.3.1; abundance Zrn peak at 485 Ma,
Zrn crystallisation between
c.
520 and 470 Ma),
the
Punta do Serrón
mafic rock (not studied in
this work) and the
Punta da Moura
mafic rock
(represented by sample GCH-23; section 7.3.4;
Zrn crystallisation peak at 508 Ma). When these
magmas were cooling and crystallising, felsic
magma (sample GCH-14; section 7.4.4) intruded
into the mafic rocks in the
Punta da Moura
area at
c.
500 Ma to form the
Punta da Moura
mingling suite.
7.4. ORTHOGNEISSES