Nagoya University
NagoyaUniversity

Provenance analysis based on clastic composition
and CHIME
age ofdetritalzircons on the Lower JurassicYamaoku Formation, Chugoku
Meuntains,SouthwestJapan
HiroshiKamikubo, Makoto Takeuchi,KazuhiroSuzuki
NkegQya Clhiversit}llapan
to clarify therole of tectonic
event. Thepresents importantinfbrmation
clastic rocks ofJapan
weresignificantly changedin Latc Triassicto Early

IntroductioniProvenanceanalysis
provenancesofpre-Cretaceous
Jurassictime2000). However, there islittle
agreement(Takeuchi,In SouthwestJapan,shallow
This study

on the tectonicsetting fbrthe changes.
Lower Jurassicsediments are sparsely distributed,clastic compositions
as indicatorof source rock and the time-spatial

marineaddresses

andaims
forundcrstanding
tectonic eventon the Southwest Japan during Early Jurassic time.
The Lower JurassicYamaoku Formation is mainly composedof alternating beds of sandstone
andvariation,

mudstonewithseveralacidicvolcanic

on lithofacies,
Studiesofmodal

layers,andcompositions

subdivided

ofsandstones,

into lower, middle
chemicalcompositions

and upper members

based

gamets andofdetrital

Cr-spinels,
andCHIME ages ofdetrital zircons were carried out intheYamaoku Formation.
Result:Sandstoneand conglomerateof the Yamaoku
Formation includeacidic volcanic,
acidicplutonic,
intermediate to basicvolcanic, sandstone, chert, mudstone
and crystalline schist fragments.Mudstone
fragmentsrarely yieldradiolarian fossils,
Modal compositions
show
decreaseof rock fragmentsand
increaseof K-feldsparstratigraphically upward,
Pyrope-richalmandine (thehighest8.9 wt.% MgO)
decreasesfrom lowerto upper member
44.1%,middle;
24.2%,upper;
6.9%). The same change is(lower;recognizedin grossular-richalmandine
highest7,3wt,% CaO) (lower;4.8 %, middle; O,5%, upper;(theO %). On the other hand,spessartine-almandine
series poor in pyrope andincreases from lower togrossular< O.25wt%)
upper member
as fo11ows;
55.1 %, 75.3%, 93.1 % in number
firequency.
Low-Ti (Ti02groupof Cr-spineldecreases
from lowerto upper member as fo11ows;11.8 %, 42.3 %, 50.0 % in nurnl)er
frequency. In addition, CHIME ages of detrital
zircon indicate
mostly ca. 2000, 250 and 180 Ma and rarely
ca. 1500, 500, 350 Ma,
Provenance of the Yamaoku Fermation and itschaBgewithage:
Detrital180 Ma zircons are likely
tohave been derivedfrom coeval acidic volcanic rocks with accumulationof the Yamaoku Formation,
becausemany acidic tuffS are interca]atedwith the Yamaoku Formation. Source rocks of 250 Ma and2000Ma zircons
could
not be identifiedfrom geologicalevidencesin this study. However, 180 Ma, 250
Ma, 1500 Ma and 2000 Ma ages were reported from granitic
rock clasts and gneissclasts in Permian toJurassic
clastic
rocksofJapan
Shibataand Adachi, 1974; Tanaka et al., 2002).PossiblePrecambrian(e.g.
high-grad¢ gnejssesincludingpyrope-richalmandine garnetsare distributedin the Sino-Korea,Yangzi,Huanan,Khanka and Breya massifs.
Low-Ti group of Cr-spineland grossular-richalmandinemighthave been derivedfrornthe
Oeyama ophiolite and ca. 300 Ma Renge metamorphic
rocks,
respectively,
as basement rocks of theYamaoku Formation.Chert and radioiarian-bearing
mudstone
werederivedfrom the Permian accretionary
complexof the AkiyoshiBelt.
On the basisof clastic compositions, the provenanceof the Yamaoku Formationchanged fromtheneighboringAkiyoshi

Belt, Oeyama

ophioiiteandRengemetamorphicrocks

associatedPrecambriancontinental basement with acidic vo]canic rocks to granitic
rocks extensively.
Reference: Adachi and Shibata(1974)
EPSL, 21,277-287.Takeuchi(2000)
Mlem. Geol,Soc,
183-I94.Tanaka et al,,(2002),lounEarthPranet,Sci,Ndgoya Uhiv.,49, 1-13,.ldpan,

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