Geology and petrology of some polymetamorphosed amphibolites and associated rocks in northeastern Taiwan: Summary

The pre-Tertiary metamorphic complex of northeastern Taiwan consists of schist, marble, gneiss, amphibolite, (meta)granodiorite, and minor serpentinized peridotite. Fault-bounded and foliated amphibolite with the assemblage green hornblende + plagioclase (An 40 to An 52 ) + epidote (Ps 4 to Ps 15 ) + sphene ± rutile ± quartz was intruded by granitic rocks about 87 m.y. ago or earlier. The most apparent thermal effects include (1) transformation of green hornblende to brown hornblende, (2) transformation of epidote to symplectic intergrowths of clinozoisite + plagioclase + quartz, (3) crystallization of clinopyroxene at the expense of hornblende, and (4) production of biotite + muscovite + K-feldspar in the partly; metasomatized amphibolites. The metamorphic-igneous complex was later intruded by thin diabasic dikes; the older of these dikes are believed to be feeders for basaltic flows and pyroclastics in the overlying lower Cenozoic formations. Some dikes may be as young as Miocene. The whole complex—basement, dike, and cover rocks—was metamorphosed under greenschist facies conditions during Pliocene-Pleistocene collision between the China and the Philippine Sea plates. Amphibolite has been partly recrystallized to the assemblage actinolite + chlorite + epidote (Ps 15 to Ps 27 ) + quartz + sphene. Both nonfoliated dike rocks and foliated basaltic flows in the Cenozoic sequence carry the asssemblage albite + actinolite + chlorite + epidote (Ps 20 to Ps 26 ) + biotite + quartz + sphene. Zeolite facies recrystallization occurred locally along fractures in some amphibolite and produced laumontite + epidote (Ps 32 ) + chlorite. Bulk-rock X-ray fluorescence (XRF) compositions of 14 amphibolites fall within the range of low-K and low-Ti tholeiite, whereas 7 diabasic dike rocks and 2 basaltic rocks of the Cenozoic sequence are alkali basalts with relatively higher TiO 2 (0.7 to 2.7 wt %), Na 2 O (1.6 to 4.0 wt %), and K 2 O (0.6 to 0.8 wt %). Thermally recrystallized amphibolites enclosed in the Upper Cretaceous granitic intrusion are enriched in SiO 2 , Al 2 O 3 , and K 2 O and depleted in total Fe, MgO, and CaO. Microprobe analyses of coexisting phases show that (1) zoning of epidote, amphibole, and plagioclase reflects changes in pressure-temperature conditions—with more pronounced fractionations characteristic of lower grades; (2) the Mg/Fe ratio of chlorite is 2.0 in amphibolite, 1.2 to 1.5 in altered amphibolite, and 0.9 to 1.0 in greenschist; and (3) the amphibolite facies hornblendes contain high pargasite-tschermakite proportions (Al 2 O 3 = 8 to 13 wt %; TiO 2 = less than 0.6 wt %; Mg/Mg + Fe ratio = 0.54 to 0.65), the thermally recrystallized brown hornblendes contain substantially more TiO 2 (1.0 to 1.9 wt %), slightly lower Si and hence higher Al IV and elevated total aluminum contents compared to the green and blue-green hornblendes, whereas the greenschist facies amphiboles are actinolitic, with Al 2 O 3 less than 5.9 wt %. Fractionation of elements between coexisting phases in the amphibolites and associated rocks are systematic and, in general, suggest a close approach to chemical equilibrium; in most cases, the exchange reactions appear to be of the ion-for-ion type, except where several structurally distinct crystallographic sites are involved in the partitioning. Comparisons with experimentally determined phase equilibria and element distributions among coexisting minerals yield the following estimates of pressure-temperature conditions for the various recognized stages of metamorphic recrystallization: (1) amphibolite facies metamorphism at pressures of about 5 kb and temperatures approaching 650 °C; (2) accompanying the granitic intrusion, potassium metasomatism and thermal metamorphism at about 700 °C, judging from the incipient breakdown of epidote-clinozoisite in the amphibolite and the formation of clinopyroxene at the expense of hornblende; and (3) Pliocene-Pleistocene greenschist facies metamorphism at 350 to 475 °C and P total of no more than about 5 kb. Cretaceous or earlier sea-floor spreading apparently generated the Suao-Nanao basaltic + ultramafic protolith; these rocks, overlain by younger sediments, were transported to and sequestered at the Asiatic continental margin, accompanied by an intermediate-pressure recrystallization event prior to or during Cretaceous time. Cretaceous calc-alkalic plutonism and thermal metamorphism attended marked convergent plate motion. Only minor (slightly alkalic) mafic magmatic activity was associated with a hypothesized early Cenozoic rifting of the Asiatic continental margin. Greenschist facies recrystallization, which apparently took place during Pliocene-Pleistocene time, is inferred to reflect the collision of the Chinese shelf + slope + rise with the western edge of the Philippine Sea plate (= Luzon arc).