Petrology and age determinations of the ultramafic (lamproitic) rocks from the Yakokut complex, Aldan Shield, Eastern Siberia

Abstract

Mineralogical, geochemical and isotopic data including K-Ar age determinations are presented for one of the largely unknown, Mesozoic ultramafic (lamproitic) occurrences of the Aldan Shield. Ultramafic, ultrapotassic rocks occur as isolated pipes or as dykes in alkaline complexes as the Yakokut complex. K-Ar dating of phlogopites from different ultramafic dyke rocks of Yakokut give cooling ages of 133.3 ± 1.3 Ma. These rocks generally contain olivine, clinopyroxene, phlogopite and spinel phenocrysts. Olivines are forsterite-rich (Fo86–94) and undeformed, suggesting that they are phenocrysts. The low Ti and Al contents of clinopyroxenes are close to those of lamproites. Phlogopite cores are rich in Al2O3 and TiO2 relative to the rims with two different zonation trends caused by distinct crystallization conditions after emplacement. Spinels are Ti-bearing magnetites and Ti-Al-magnesiochromites with high Cr/(Cr+Al) ratios (> 0.9), indicating their crystallization from a lamproitic melt.Geochemically, the rocks are ultrabasic — basic with high mg# values, low Ca, Al, Na and strong enrichments of Rb, Ba and K. Their CaO/Al2O3, Zr/Nb or Ba/Sr ratios indicate their lamproitic nature and origin in the subcontinental lithosphere of a depleted mantle source which had undergone metasomatic enrichment. As the rocks all show strong negative Nb anomalies and low Ti, Y and Yb contents, the enrichment is attributed to subduction zone fluids. The Sr-Nd-data (87Sr/86Sr0 0.70573−0.70605 ± 0.00003; ɛNd − 10.2 ± 0.7) indicate the origin by partial melting of a heterogeneous mantle source with relatively low Rb/Sr ratios and an early enrichment. The evolution model comprises a depletion of ‘basaltic elements’, leading to a harzburgitic source which was enriched by an early LREE contribution during the stabilization of the Archaean to Proterozoic Aldan Shield. It is further suggested that the Mesozoic northwesterly directed subduction of the Ochotsk-Chukotsk belt influenced the subcontinental lithosphere underneath the Aldan Shield, leading to the observed subduction-related signature of the Yakokut lamproites.

Mineral chemistry and geochronology of the potassic alkaline ultramafic Inagli complex, Aldan Shield, eastern Siberia

“…Following the procedure outlined above, MgO contents as high as 26,9 wt.% are required for the parent liquid of Inagli. This is quite high, nevertheless it seems reasonable, as lamproites from the Yakokut complex (about 40 km E of Inagli) have MgO contents of up to 25.7 wt.% (Mg# 83.9; Mg#o~ 94.0; Mues-Schumacher et al, 1995). The above calculation using such a lamproite melt crystallizing an olivine of Mg# 95.7 results in an even higher MgO content of 28.2 wt.% from such a lamproitic parental melt.…”

“…Kostyuk et al (1990). The intrusion of the Inagli complex is thus more or less contemporaneous with the intrusion of other alkaline complexes in the Aldan-Shield (Mues, 1993;Mues-Schumacher et al, 1995). Alternative models for the genesis of lnagli.…”

Fingerprints of Kamafugite-Like Magmas in Mesozoic Lamproites of the Aldan Shield: Evidence from Olivine and Olivine-Hosted Inclusions

“…Since the estimated temperature of LAM generation (above 1350 • C) is significantly higher than required for KAM generation, we suggest that there was a deep thermal flux, which may also have supplied K and Si to the source [19] ( Figure 12C). The suggestion of substantial input of mobile alkaline and alkali-earth elements in the source during the Mesozoic is also supported by a high dispersion of Sr and Nd isotope compositions of the Aldan lamproites, with Sr being generally more primitive than Nd [12,30,36,97,98]. This modified SCLM, veined by a clinopyroxene + phlogopite + orthopyroxene ± olivine assemblage, along with thermal and tectonic (continuing extension) conditions were sufficient for the generation of ultrapotassic silica-saturated melts of lamproitic affinity [6,67,88,91,99] (Figure 12D).…”

“…Lamproite magmatism occurred in two pulses, in the Late Jurassic and Early Cretaceous [11]. Despite this temporal diversity, the majority of Aldan lamproites are considered to be products of a similar magmatic process, with minor chemical differences between occurrences (e.g., variations of MgO, CaO and Al 2 O 3 contents) [12,16,26,34,36].…”

Trace element and Sr-Pb-Nd-Hf isotope evidence for ancient fluid-related enrichment in the source region of Aldan Shield lamproites

“…Second, olivine and diopside phenocrysts have slightly lower abundances and lower Mg# (88-90). More extensive petrographic descriptions can be found in Mahotkin et al (1989), Mues et al (1995, and Vladykin (1997).…”