tübinger geowissenschaftliche arbeiten (tga) - TOBIAS-lib ...

3.3.2 Major and trace elements
Major and trace element concentrations of all samples are compiled in Appendix A,
Tab. A2. Plutonic rocks of the South Tien Shan (STS) were identified as monzogranites,
and the subvolcanic rocks as granodiorites (Fig. 3.5a). Metavolcanic rocks of the
northern Pamirs (Altyndara valley) have basaltic to rhyolitic compositions, batholiths of
the Lake Karakul area are monzogranites and granodiorites, whereas most plutons of
the Central Pamirs (CP) and the Rushan Pshart Zone (RPZ) are monzogranites. Further
sampled rock types in the Central Pamirs are a leucogabbro (96M9a) and a latiteandesite
(A96S1b).
Igneous rocks from the STS belong to sub-alkaline respectively high-K calc-alkaline,
mainly peraluminous I-types (Fig. 3.5b, h), and only the northernmost sampled rocks
TS1 and TS2 are transitional to S-type series. The latter two samples imply a higher
degree of crustal contamination as indicated by the lack of hornblende, more K-feldspar
megacrysts, and the higher A/CNK ratio and SiO 2 content (Fig. 3.5b). All rock samples
are interpreted as volcanic arc related (Fig. 3.5f-i), but with probably more fractionated
rock types to the northeast. Chondrite-normalised rare earth element (REE) patterns of
the South Tien Shan samples (Fig. 3.6a) show strong fractionation of the LREE
(La N/Sm N = 4-6) and only a weak negative Eu-anomaly (Eu* was determined from
interpolation of REE patterns between Sm and Tb). The HREE are most fractionated in
samples TS12a,b and less fractionated in the subvolcanic rock samples TS18a, TS20a.
The North Pamirs metavolcanic rocks from the Altyndara valley have low- to medium-K
character (Fig. 3.5c) and are metaluminous (Fig. 3.5b). Although all samples show a
clearly calc-alkaline trend (Fig. 3.5d), the basalt samples plot into the MORB field (Fig.
3.5e). The rock-types with MORB affinity were formed in an oceanic or early arc setting,
whereas all other samples represent normal volcanic arc rocks (Fig. 3.5f). This is also
reflected in the chondrite-normalised REE patterns (Fig. 3.6b): the oceanic or early-arc
volcanic rocks show flat, typical MORB patterns; normal arc samples and samples with
increasing SiO 2-contents show steeper LREE fractionation patterns. Only the samples
AD2a and AD6b display a distinctly negative Eu-anomaly.
The Lake Karakul batholiths are peraluminous, mainly S-type monzogranites (Fig. 3.5b),
and have high K-contents. In context with their petrological features, they are classified
as post-tectonic granitoids with an arc to late-/post-collisional geochemical signature
(Fig. 3.5f-i). The rocks show a consistent strong fractionation from La to Sm and the
magnitude of the negative Eu-anomaly (Eu/Eu* = 0.62-0.74) increases with increasing
SiO 2-contents (Fig. 3.6b).
The leucogabbro 96M9a (dark grey circle) intrudes schists and gneisses in the eastern
part of the Muzkol dome. It has low K-contents (Fig. 3.5c), a metaluminous character
(Fig. 3.5b), and seems to be secondarily enriched in SiO 2. The REE pattern is generally
enriched with La N/Lu N = 20.6 and shows a positive Eu-anomaly (Eu/Eu* = 1.2),
probably due to plagioclase enrichment.
The Triassic/Jurassic monzogranites (dark grey circles) are heterogeneous in their
major- and trace element contents. Sample L96M25a is a metaluminous I-type
monzogranite with high K-contents like the other samples of this group (Fig. 3.5a-c).
The rock is interpreted as volcanic arc related (Fig. 3.5f-i) with an overall depletion of
HREEs and a weakly negative Eu-anomaly (Fig. 3.6c). The two Jurassic samples L96A9
29