Paleozoic cherts outcrops were widely developed in the Mianlue tectonic belt

which could reveal the sedimentary environment and tectonic background of southern Qinling orogenic belt. In Sanchazi area

the cherts were found in the Sanhekou Group of the Middle Devonian. Geochemical analyses showed that Sanchazi cherts were high in SiO

2

(94.04%-98.08%

with a mean value of 96.92%) and low in Al

2

O

3

and TiO

2

and were relatively high in Ba and U

but was relatively low in Zr for trace elements. The U/Th and Ba/Sr ratios were relatively high too. ΣREE content was significantly low (7.61-51.04 μg/g

with an average valve of 2251 μg/g). PAASnormalized REE distribution pattern inclined to the left

rich in HREE and with positive Eu anomaly and negative Ce anomaly. The Al/(A1+Fe+Mn)

Fe/Ti and (Fe+Mn)/Ti values indicated the origin of hydrothermal sedimentation of the cherts. Al/(Al+Fe)

 Al

2

O

3

/(Al

2

O

3

+Fe

2

O

3

+MnO)

Al2O3/(Al

2

O

3

+Fe

2

O

3

)

δCe and (La/Ce)N values indicated that the cherts were mainly formed in the continental margin. The values of (La/Lu)N

(La/Yb)N and δEu could be related to the hydrothermal activities controlled by the deep fractures. Th/U

U/Th

δU values indicated the cherts were formed in the anoxic depositional environment. The above-mentioned geochemical evidences inferred that Sanchazi cherts were formed in a sedimentary environment of rifted basin at continental margin in the north of South China Block during the Paleozoic. During the rifting period

while a series of extensional structures

such as rift or fault basin and syngenetic faults

were formed

accompanied magma activities could provide heat source for the thermal water

and the regional syngenetic faults provided the channel for upwelling of the thermal water. Hence

the convective hot water became rich in SiO

2

through the leaching and dissolving of chemical composition from surrounding strata

and contributed to the forming of hydrothermal sediment cherts of different geologic ages in the Mianlue tectonic belt.