Geology Reference
In-Depth Information
respectively. In some of the most MgO
rich rocks of Alto Paranaiba igneous
province (e.g. Tres Ranches intrusions), the highest concentration of LREE
(La = 402 ppm) is observed. The highest enrichment of HREE (Lu = 0.49 ppm) is
obtained in rocks of Sao Gotardo, which has the lowest MgO content. This high
HREE indicates the effect of fractional crystallization in the evolution of the rocks.
Gibson et al. observed that all the ma
-
c potassic rocks are more enriched in LREE
compared to MREE with La/Sm ratio lying in the range of 6
12. They observed
that the rocks of Tress Ranchos intrusion exhibit the most extreme enrichment in
LREE related to MREE and has the highest La/Yb ratios (230). The lamproitic
rocks however, have the lowest La/ Yb ratios (70
-
100) of all the ma
c potassic
-
rocks.
The negative Sr anomaly in this province is very similar to that of the rocks in
many lamproitic localities, but presence of a trough at K is quite distinct from the
patterns displayed by many lamproitic, kimberlitic and kamafugitic provinces
(Fig.
5.27
). The rocks of this province show absence of any signi
cant depletion in
such HFS elements as Nb and Ta in the samples of Alto Paranaiba igneous prov-
ince, which contrasts with the rocks of Roman province and Leucite Hills. The Nb
content (96
364 ppm) of the K-rich rocks of this province is similar to those of the
West Kimberley lamproites and East African kamafugitic rocks. The Zr content of
the rocks of Alto Paranaiba Igneous Province (322
-
1,451) is lower than that of the
potassic rocks of Leucite Hills and West Kimberley. The rocks of this province do
not show any distinct peaks of Zr and Hf on the normalized multi-element diagram.
Gibson et al. observed that the abundance of REE of the Alto Paranaiba igneous
province is similar to that in most enriched lamproites of West Kimberley and the
Italian kamafugitic rocks. Compared to the rocks of this province, kamafugitic
rocks of East Africa have much lower REE abundance.
-
5.9 Synthesis of Trace Element and Isotopic Data
by Nelson (1992)
The potassium-rich silica-undersaturated ma
c and ultrama
c igneous rocks can be
subdivided into a subduction-related
“
orogenic
”
sub-group and an
“
anorogenic
”
sub-group that are con
ned to stable continental settings (Nelson 1992). According
to him, representatives of both sub-groups possess trace element and isotopic sig-
nature consistent with the contamination of their magma sources by incompatible
element-rich and isotopically evolved
components. He argued that
these metasomatic components are principally derived from subducted lithosphere,
including subducted sediments. Nelson concluded that examples of orogenic
potassic magmatism (e.g. Italian potassic rocks, Spanish lamproites, Sunda arc
leucitites) have trace element and Sr, Nd and Pb isotopic characteristics, consistent
with the contamination of their mantle sources by a component derived from marine
sediments. He thought potassic magmas belonging to anorogenic subgroups have
“
metasomatic
”
Search WWH ::
Custom Search