On the issue of alkaline-ultrabasic rocks localization regularity in the kimberlite province Lunda (Angola)

Ostafiychuk N.N., Zorin V.N., Podvysotskiy V.T.

State technological university, Zhytomyr, Ukraine

Previously a new data about a picritic porphyrites and an alkaline basaltoids in a diatreme facies which are kindred to the kimberlitic rocks and first revealed and examined in this region was given by us (Podvystskiy, Ostafiychuk, 2012).
The signs of high alkalinity of the rocks studied by us testify to the high depth of origin of their melts in a mantle and, consequently, they are probably associated to the blocks of lithosphere and the tectonic structures, same as kimberlites of this region.

The paragenetic associated lineaments which create a typical structural pattern are dedicated with an analyzing of detailed aeromagnetic surveying results (ATS BHP Billiton) and the decrypting satellite photographs. The location of the famous kimberlitic pipes and kindred rocks to it is regarded with the association to the defined structural paragenetics. There is assumed all assignable basic tectonic faults were founded in Precambrian yet. The regional character of magnetic field conforms to the big Precambrian structures, and the more small lineaments meet the low amplitude faults, the activation of which was in later period. There are marked out the next structural paragenesises: the Precambrian, the Cretaceous (Kimberlitic) and the Postkimberlitic.

The Precambrian structural paragenesis. There are three regions (north-west, central and south-east) within the range of studied area with the famous Katoka and Kamutue kimberlitic fields. They are marked out by the character of magnetic field. The north-west and especially south-east regions have the sharp aged lenticular and stripy pattern of magnetic field. The stripes have clear north-west course and interpolated as large regional faults by us. The direction of the stripes is in line with Lukapa zone. The large lenticular anomalies of magnetic field join to the faults directly. The anomalies conform to duplexes in tectonic interpretation. The long axes of the duplexes are regularly turned by clockwise relative to the course of the striped anomalies. It evidence of the left-side shift along the anomalies on the final stage of the Precambrian folding (cofolding structural paragenesis). The central part of the region has more tranquil, less gradient character of regional magnetic field. Its length is about 50 km. The lineaments with north-west and north-east courses stand out in the central part. The location (formation) of the lineaments is associated with the Precambrian stage of development. But it was activated in later period.

The Cretaceous (Kimberlitic) structural paragenesis. The north-west and west-north-west courses lineaments stand out sharply against the magnetic field with the gradient zones and are interpreted by us as the system of the low amplitude conjugate displacements with right and left kinematic. In our opinion, the activation of this tectonic faults occurred in the Cretaceous stage of the platform development because the famous kimberlitic pipes of the region are placed on the intersection of this faults. The probable structural control of the kimberlitic location is established. As known, the kinematic of the conjugate right and left displacement is determined by the block movement ratio on obtuse and acute angles of the fault intersections. The signs of the lineament kinematic divide by obtuse angels and they meet by acute angels. The low amplitude faults with the north-west courses have right-shift kinematic and sided with main faults with angel about 15-20 degrees. The faults with west-north-west courses have left-shift kinematicand sided with main north-west tectonic faults with angel about 60 degrees. The structural pattern like this is typical for the wide zones of the right displacement on the major north-west direction. This data enable to suppose there were active movements with changing of the kinematic sign on the said zones in the Cretaceous. The review structural diagram was made up with the findings. It shows the latitudinal zones of the tension which controls the regions of kimberlitic pipes development.

The Postkimberlitic structural paragenesis. This youngest structural paragenesis is expressed with system which consists of the conjugate west-north-west course lineaments and low amplitude faults with submeridian orientation. The movements by this tectonic faults took place against the big tectonic raise with forming big graben (faults or tensile zones) meridian orientation. This data confirm geomorphological finding.

Thereby, it’s possible to do the next conclusions. All numerous Angola kimberlites displays are associated to the Lukapa passage but the commercial kimberlites were found only in the Lunda province where Lukapa structure crosses Kassai craton. The meridian location of the famous kimberlitic fields may connect with both the more simple conditions of the pipes finding with the scope of the deep-laid (to the base) river valleys and the big river valleys laying by the submeridian fault zones, which are controlled the kimberlite location. Evidently, both case occur because most anomalies on concessions, which was found as the results of aeromagnetic surveying by experts of ATS BHP Billiton, have a bent for Chikapa river valley. The certification of the located on watersheds geophysical anomalies confirms this conclusion.

Reference.

Podvysotskiy V.T., Ostafiychuk N.N. A New Data about Alkaline-Ultrabasic Volcanism in the Kimberlite Province Lunda (Angola). In.: 29^th International Conference on Ore potential of Alkaline, Kimberlite and Carbonatite Magmatism and School “Alkaline magmatism of the Earth. Sudak – Moscow, 14-22 september 2012. GEOHKI RAS, 2012. P. 112-114.