A STABLE-ISOTOPE STUDY OF ANOROGENIC MAGMATISM IN EAST CENTRAL-ASIA

A stable isotope study of 168 plutonic igneous rock and five water sam ples from Transbaikalia, East Asia, has been made, including 318 whole -rock and mineral O-18/O-16 analyses and 14 D/H analyses. This represe nts theeJrst detailed isotopic study of the enormous Phanerozoic K-ric h (mostly anorogenic) granitoid belts of this region, which are thousa nds of kilometers in length and span an age range of > 250 Ma. Of the five main intrusive suites, emplacement of the youngest (Permo-Triassi c) was accompanied by intense meteoric-hydrothermal activity, locally causing extreme O-18 depletion (delta(18)O(feldspar) < -12). This refl ects the very low delta(18)O of the water involved in these systems, w hich probably had delta(18)O < -20 and delta D < -150, consistent with the high paleolatitude of Transbaikalia in the early Mesozoic (simila r to 80 degrees N). Despite local post-emplacement, hydrothermal O-18- depletion effects near Permo-Triassic and younger plutons, the variati on of magmatic isotopic composition in the five intrusive suites, in s pace and time, can be clearly discerned using the O-18/O-16 variation in phases such as quartz and sphene that are resistant to sub-solidus exchange. A procedure for analyzing O-18/O-16 in sphene using a laser fluorination technique is described: analysis of samples as small as 0 .4 mg (including single crystals of sphene from granitoids) is possibl e and provides an effective way to estimate the magmatic delta(18)O va lue of plutonic igneous rocks. Most sphene and quarts delta(18)O value s vary by 1.0-2.0 parts per thousand within each of the five main intr usive suites in Transbaikalia (ranging in age from mid-Paleozoic to Me sozoic), and are uniform both within individual plutons and among plut ons of the same suite separated by tens or hundreds of kilometers. How ever, each suite has a unique range in O-18/O-16, indicating that, on a regional scale, the magmatic delta(18)O values of these granitoids d ecreased progressively in similar to 1 parts per thousand decrements f rom similar to+10 in the earliest group to similar to+6 in the younges t. This progression was accompanied by increases in the concentration of elements such as K and Zr, and decreases in the concentration of el ements such as Sr and Ba. These systematics require large-scale deep c rystal melting and mixing processes to generate the compositional unif ormity of individual plutons and groups over such wide areas, and also a progressive hybridization of the crust with alkalic, mantle-derived magmas to generate syenites and granites with progressively lower del ta(18)O values. This process may be a hallmark of anorogenic granitoid petrogenesis and the intracontinental cratonization process in genera l, and also represents an important (though largely cryptic) crystal g rowth mechanism.