Abstract The orogenic continental crust of accretionary and collisional belts worldwide is dominated by felsic and metasedimentary rocks, which show variable responses to high-grade metamorphism. Transformation of felsic rocks is commonly limited as compared to that of the enclosed mafic rocks (including eclogites sensu stricto), which is widely attributed to availability of H 2 O–CO 2 fluids, kinetically controlled growth of high-grade assemblages, and their preferential preservation in metabasites more competent to rehydration. We report on the results of studies of the geochemical behaviour of zircon (trace-element, U–Pb, Lu–Hf and δ 18 O) in three felsic samples (two metagranitoids and one paragneiss), which are spatially (geographically and at the outcrop-scale) juxtaposed with mafic eclogites within the North Muya block (Neoproterozoic Baikalides, northern Central Asian Orogenic Belt). The data imply that metagranitoids and metasediments within the buried continental lithosphere might follow a single subduction-related P – T – t trend, whereas contrasting degrees of mineralogical and zircon transformation were governed by mineral buffer reactions in the absence of external fluids. The latter was significant only in an H 2 O-enriched protolith of metasediments. The formation of 18 O-depleted zircon recrystallization rims together with Mn enrichment of garnet rims indicate a distinct metamorphic stage without or with minor localized fluid infiltration, most likely, related to peak temperature conditions during collision.

ABSTRACT Spatial distributions of widespread igneous arc rocks and high-pressure–low-temperature (HP/LT) metamafic rocks, combined with U-Pb maximum ages of deposition from detrital zircon and petrofacies of Jurassic–Miocene clastic sedimentary rocks, constrain the geologic development of the northern and central Californian accretionary margin: (1) Before ca. 175 Ma, transpressive plate subduction initiated construction of a magmatic arc astride the Klamath-Sierran crustal margin. (2) Paleo-Pacific oceanic-plate rocks were recrystallized under HP/LT conditions in an east-dipping subduction zone beneath the arc at ca. 170–155 Ma. Stored at depth, these HP/LT metamafic blocks returned surfaceward mainly during mid- and Late Cretaceous time as olistoliths and tectonic fragments entrained in circulating, buoyant Franciscan mud-matrix mélange. (3) By ca. 165 Ma and continuing to at least ca. 150 Ma, erosion of the volcanic arc supplied upper-crustal debris to the Mariposa-Galice and Myrtle arc-margin strata. (4) By ca. 140 Ma, the Klamath salient had moved ~80–100 km westward relative to the Sierran arc, initiating a new, outboard convergent plate junction, and trapping old oceanic crust on the south as the Great Valley Ophiolite. (5) Following end-of-Jurassic development of a new Farallon–North American east-dipping plate junction, terrigenous debris began to accumulate as the seaward Franciscan trench complex and landward Great Valley Group plus Hornbrook forearc clastic rocks. (6) Voluminous deposition and accretion of Franciscan Eastern and Central belt and Great Valley Group detritus occurred during vigorous Sierran igneous activity attending rapid, nearly orthogonal plate subduction starting at ca. 125 Ma. (7) Although minor traces of Grenville-age detrital zircon occur in other sandstones studied in this report, they are absent from post–120 Ma Franciscan strata. (8) Sierra Nevada magmatism ceased by ca. 85 Ma, signaling transition to subhorizontal eastward underflow attending Laramide orogeny farther inland. (9) Exposed Paleogene Franciscan Coastal belt sandstone accreted in a tectonic realm unaffected by HP/LT recrystallization. (10) Judging by petrofacies and zircon U-Pb ages, Franciscan Eastern belt rocks contain clasts derived chiefly from the Sierran and Klamath ranges. Detritus from the Sierra Nevada ± Idaho batholiths is present in some Central belt strata, whereas clasts from the Idaho batholith, Challis volcanics, and Cascade igneous arc appear in progressively younger Paleogene Coastal belt sandstone.