Petrology, Geochemistry and Geochronology of Lamprophyres from the UHP Makbal Complex, NW-Kyrgystan

The Makbal Complex is located within the western Tien Shan mountain range in NW-Kyrgyzstan. It’s central part comprises high-pressure (HP) and ultrahigh-pressure (UHP) metasedimentary and metabasaltic rocks of continental and oceanic origin, respectively.

Within the eastern part of the Makbal complex, abundunt 0.5 to 5 meter wide NW-SE oriented sills occur mainly within the Neldy group, but were also found in the Chymynsai and Kaindy groups.

The sampled rocks are altered to different extents with a dark to medium gray-green fine grained matrix comprising mainly chlorite and one to three millimeter sized porphyroclasts of amphibole, biotite, feldspar and carbonate, the latter most likely of secondary origin. Chlorite is not only the dominating matrix phase due to low grade alteration, it often replaces other mafic minerals such as amphibole, biotite and clinopyroxene. Although the majority of samples are highly altered, some phenocrysts are still fresh and include: (1) amphibole (kaersutite), (2) Mg-rich augite, (3) biotite of intermediate Fe-Mg content, (4) plagioclase of andesine to labradorite composition, and in some cases (5) K-feldspar. Based on the observed porphyritic texture and distribution of observed phenocrysts, the dikes can be classified as lamprophyres belonging mainly to the spessartite and to a lesser extent to the minette and vogesite groups.

Within the Nb/Y–Zr/Ti as well as the TAS diagrams (Pearce, 1996, Le Bas et al, 1986), the samples plot in the basalt, basaltic andesite, trachyandesite, and andesite fields. They all fall into the subalkaline field and most follow a shoshonitic or high-K calcalkaline trend in the SiO₂-K₂O diagram (Peccerillo and Taylor 1976). According to the Ti-Zr classification diagram after Pearce and Cann (1973) and the Nb/Yb−Th/Yb diagram (Pearce 2008), the lamprophyres were clearly emplaced within a compressional/continental arc setting. The chondrite normalized rare earth element pattern display a 100 times enrichment in light rare earth elements and a nearly constant 10 to 20 times enrichment of the middle and heavy rare earth elements excluding a deep-seated garnet bearing mantle as source of the lamprophyre melt. The patterns neither show a pronounced negative Nb-Ta anomaly nor any Eu anomaly.

Zircons could be extracted from an altered lamprophyre sample with kaersutite and plagioclase phenocrysts. The zircons are elongated with magmatic oscillatory zoning in CL image. The weighted mean ²⁰⁶Pb/²³⁸U age is 457 +/- 1 Ma, but a trend towards younger ages down to 440 Ma is observed. This age is similar to the intrusion age of a calcalkaline granodiorite body exposed approximately 5 to10 km to the west of the lamprophyre dikes. Both magmatic intrusives post-date the UHP event in the Makbal complex and bear important information to understand the full tectonic evolution. A genetic relationship of the lamprophyres and mafic enclaves found within the granodiorite is postulated.