A stable isotope study of anorogenic magmatism in East Central Asia

A stable isotope study of 168 plutonic igneous rock and five water samples from Transbaikalia, East Asia, has been made, including 318 whole-rock and mineral O-18/O-16 analyses and 14 D/H analyses. This represents theeJrst detailed isotopic study of the enormous Phanerozoic K-rich (mostly anorogenic) granitoid belts of this region, which are thousands of kilometers in length and span an age range of > 250 Ma. Of the five main intrusive suites, emplacement of the youngest (Permo-Triassic) was accompanied by intense meteoric-hydrothermal activity, locally causing extreme O-18 depletion (delta(18)O(feldspar) < -12). This reflects the very low delta(18)O of the water involved in these systems, which probably had delta(18)O < -20 and delta D < -150, consistent with the high paleolatitude of Transbaikalia in the early Mesozoic (similar to 80 degrees N). Despite local post-emplacement, hydrothermal O-18-depletion effects near Permo-Triassic and younger plutons, the variation of magmatic isotopic composition in the five intrusive suites, in space 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 possible and provides an effective way to estimate the magmatic delta(18)O value of plutonic igneous rocks. Most sphene and quarts delta(18)O values vary by 1.0-2.0 parts per thousand within each of the five main intrusive suites in Transbaikalia (ranging in age from mid-Paleozoic to Mesozoic), and are uniform both within individual plutons and among plutons of the same suite separated by tens or hundreds of kilometers. However, 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 decreased progressively in similar to 1 parts per thousand decrements from similar to+10 in the earliest group to similar to+6 in the youngest. This progression was accompanied by increases in the concentration of elements such as K and Zr, and decreases in the concentration of elements such as Sr and Ba. These systematics require large-scale deep crystal melting and mixing processes to generate the compositional uniformity 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 delta(18)O values. This process may be a hallmark of anorogenic granitoid petrogenesis and the intracontinental cratonization process in general, and also represents an important (though largely cryptic) crystal growth mechanism.