Time and Nature of Rb-Sr Isotopic System Disturbance in North Kazakhstanian Granites

K. N. Shatagin Inst. of Geology of Ore Deposits (IGEM) RAS, 35 Staromonetny per., Moscow, 109017 Russia


Caledonian granites of North Kazakhstan underwent a number of superimposed events. Various kinds of alteration of minerals resulted in a disturbance of their Rb-Sr isotopic system. A combination of different chemical stability and different closure temperature of the studied minerals made the final isotopic picture complex.

A study of leucogranites of Zolotonosha pluton (398±3Ma) showed different degree of a response to the late (300 Ma) hydrothermal event for biotite and K-feldspar. During the alteration biotite was chemically stable and retained its Rb-Sr system closed while the K-feldspar was altered by a fluid and its Rb-Sr system equilibrated with the fluid. Isochron on the fractions of K-feldspar with various specific gravity points to the time of the alteration event, in contrast, the apparent age of biotite is close to the time of the pluton emplacement.

The same complex picture is characteristic for granites of Orlinogorsky pluton. But there is a difference -- isochron on the fractions of biotite with different specific gravity has negative initial Sr-isotope composition. The feature is characteristic of the two-component mixture. One of the probable explanations to the fact is that the biotite fractions contain different amount of magmatic grains and late postmagmatig overgrowths on them. The latter is younger and has lower Rb/Sr-ratio relative to the magmatic one.

Rb-Sr data on whole rock samples and mineral separates from Zerenda batholith suggests a number of alteration pulses in the history of a relatively small outcrop. After an emplacement of the granites 450 Ma they were cut by felsic dikes
and pegmatitic veins at 410-420 Ma. The emplacement of pegmatite resulted in crystallization of a secondary epidote in the granites. The mineral has high Sr-isotope composition of 0.712(?). Later, some 280-300 Ma, the rocks of the studied outcrop underwent the hydrothermal event (probably, simultaneously with granites of Zolotonosha and Orlinogorsky plutons). Reaction of the granite rock-forming minerals with a hydrothermal fluid equilibrated Sr-isotope composition in they, but the secondary epidote and (surprisingly) subordinate apatite retained their isotopic signatures. The epidote was probably chemically stable and its Rb-Sr system possibly has high closure temperature, but the same is not true for the apatite. An explanation may be that a temperature during the last event was lower than the closure temperature for the Rb-Sr system in the apatite, and, as it is shielded by the biotite from the remaining rock, the mineral kept its original isotope composition.