Igneous rocks collected from the Ulugh Muztagh, 200 km south of the northern rim of the Tibetan Plateau (36¿28'N, 87¿29'E), form intrusive and extrusive bodies whose magmas were produced by partial melting of upper-crustal, primarily pelitic, source rocks. Evidence for source composition includes high initial 87Sr/86Sr ratios (~0.711 to 0.713), 206Pb/204Pb ratios of 18.72, 207Pb/204Pb of 15.63 and 208Pb/204Pb of 38.73. The degree of melting in the source region was increased by significant heating via in situ decay of radioactive nuclides; a reasonable estimate for the heat production rate in the source is 3.9¿10-6 W/m3. The crystallization ages and cooling ages et al., 1989> of the earliest intrusive rocks within the suite suggest crustal thickening began in the northern Tibetan Plateau before 10.5 Ma, with maximum average unroofing rates in this part of the Tibetan Plateau for the period between 10.5 and 4 Ma at approximately <2 mm/yr. The Ulugh Muztagh flows are at the northern edge of a widely distributed field of Plio-Pliestocene volcanic rocks in the north-central Tibetan Plateau. The crystally derived rocks described here are an end-member component of a wide mixing zone of hybrid magmas; the other end-member forms mantle-derived, potassic basanites and tephrites exposed in the central section of the Plio-Pleistocene field. The compositional trends in these belts strike east-west, at high angle to the N30E strike of the Plateau itself. Consideration of the chemical data and published geophysical data argue that the sub-Plateau mantle is mechanically detached from the overlying continental lithosphere, and that in this section of the plateau the thermal structure of the asthenosphere is not responsible for the formation or maintenance of the plateau's topography. ¿ American Geophysical Union 1990