DEFINITION OF CHINA’S DONGWANZI OPHIOLITE, OLDEST COMPLETE OPHIOLITE

The North China, Tarim, and Yangtze cratons form the bulk

of Precambrian rocks in China, covering an area of about

15.5 million square miles (4 million km2), or about 40 percent

of China. The North China craton is approximately 65.6

million square miles (1.7 million km2) in area and forms a triangular

shape covering most of north China, the southeastern

part of northeast China, Inner Mongolia, Bohai Bay, Northern

Korea, and part of the Yellow Sea regions. It is divided

into the eastern and western blocks and the central orogenic

belt. The western block and central orogenic belt are separated

by the younger Huashan-Lishi fault in the south and by

the Datong-Duolun fault in the north. The central orogenic

belt and eastern block are locally separated by the younger

Xinyang-Kaifen-Jianping fault. Rock formation ages range

China’s Dongwanzi ophiolite, oldest complete ophiolite 71

from 2.7–2.5 Ga in the eastern and western blocks and 2.5

Ga for the central orogenic belt. Detrital zircons from the

eastern block have been dated at 3.8–3.5 Ga and are the oldest

ages obtained from the North China craton. The average

lithospheric thickness ranges from 50 miles (80 km) in the

east to 100 miles (160 km) in the west. The difference in

thickness between the blocks is reflected in the lack of a thick

mantle root in the eastern block.

The Zunhua structural belt is located in the northern part

of the central orogenic belt, in the eastern part of the Hebei

province, and covers an 81 × 12.5 square mile (130 × 20 km2)

area. Numerous mafic and ultramafic boudins have been identified

within the Zunhua structural belt, including typical

ophiolitic assemblages, which in turn include cumulate ultramafic

rocks and pillow lavas, podiform chromite, well-preserved

sheeted dikes, pyroxenite, wehrlite, and partly

serpentinized harzburgite. These tectonic ultramafic blocks

are part of a complex mélange zone found along with the

Dongwanzi ophiolite and probably represent deeper-crustal

parts of the ophiolite. Podiform chromites are found within

harzburgite tectonite and dunite host rocks and contain nodular

and orbicular textures. These types of chromite textures

are found exclusively in ophiolites and are thought to have

formed during partial melting of the flowing upper mantle.

Chromites from the ophiolite have been dated by the Re-Os

method to be 2,547 ± 10 million years old.

The Dongwanzi ophiolite belt is located in the northeast

part of the Zunhua structural belt and has been interpreted to

represent a large ophiolitic block within the Zunhua ophiolite

mélange. The Dongwanzi ophiolite belt is about 31 miles

(50 km) long and from three to six miles (5–10 km) wide and

preserves the upper or crustal part of the ophiolite suite, with

deeper sections of the ophiolite being preserved in related

blocks. The ophiolite is broken into three main thrust slices,

including the northwest belt, central belt, and the southeast

belt. All of the belts are metamorphosed to at least greenschist

facies and typically amphibolite facies, with conditions

approaching granulite in the west.

A high-temperature shear zone intruded by the 2.4-Gaold

diorite and tonalite marks the base of the ophiolite.

Exposed ultramafic rocks along the base of the ophiolite

include strongly foliated and lineated dunite and layered

harzburgite. Aligned pyroxene crystals and generally strong

deformation of serpentinized harzburgite resulted in strongly

foliated rock. Harzburgite shows evidence for early high temperature

deformation. This unit is interpreted to be part of

the lower residual mantle, from which the overlying units

were extracted.

The cumulate layer represents the transition zone between

the lower ultramafic cumulates and upper mafic assemblages.

The lower part of the sequence consists of orthocumulate

pyroxenite, dunite, wehrlite, lherzolite and websterite, and

olivine gabbro-layered cumulates. Many layers grade from

dunite at the base, through wehrlite, and are capped by

clinopyroxene. Some unusual ultramafic cumulate rocks in the

central belt include hornblende-pyroxenites, hornblendites,

and plagioclase-bearing pyroxene hornblendites. Basaltic dikes

cut through the cumulates and are similar mineralogically and

texturally to dikes in the upper layers.

The gabbro complex of the ophiolite is up to three miles

(5 km) thick and grades up from a zone of mixed layered

gabbro and ultramafic rocks to one of strongly layered gabbro

that is topped by a zone of isotropic gabbro. Thicknesses

of individual layers vary from centimeter to meter scale and

include clinopyroxene and plagioclase-rich layers. Layered

gabbros from the lower central belt alternate between finegrained

layers of pyroxene and metamorphic biotite that are

separated by layers of metamorphic biotite intergrown with

quartz. Biotite and pyroxene layers show a random orientation

of grains. Coarse-grained veins of feldspar and quartz

are concentrated along faults and fractures. Plagioclase

feldspar shows core replacement and typically has irregular

grain boundaries. The gabbro complex of the ophiolite has

been dated by the U-Pb method on zircons to be 2,504 ± 2

million years old.

The sheeted dike complex is discontinuous over several

kilometers. More than 70 percent of the dikes exhibit one-way

chilling on their northeast side. Gabbro screens are common

throughout the complex and increase in number and thickness

downward marking the transition from the dike complex to

the fossil magma chamber. In some areas, the gabbro is cut by

basaltic-diabase dikes, but in others it cuts through xenoliths

of diabase suggesting comagmatic formation.

The upper part of ophiolite consists of altered and

deformed pillow basalts, pillow breccias, and interpillow sediments

(chert and banded iron formations). Many of the pillows

are interbedded with more massive flows and cut by

sills; however, some well-preserved pillows show typical

lower cuspate and upper lobate boundaries that define stratigraphic

younging. Pyroxenes from pillow lavas from the

ophiolite have been dated by the Lu-Hf method to be 2.5 billion

years old, the same age as estimated for the gabbro and

mantle sections.

Prior to the discovery of the Dongwanzi ophiolite, portions

of several Archean greenstone belts had been interpreted

to contain dismembered or partial ophiolites, but none of

these contain the complete ophiolite sequence. Several welldocumented

dismembered Archean ophiolites have three or

four of the main magmatic components of a full ophiolite.

Archean greenstone belts have a greater abundance of accreted

ophiolitic fragments compared to Phanerozoic orogens,

suggesting that thick, relatively buoyant, young Archean

oceanic lithosphere may have had a rheological structure

favoring delamination of the uppermost parts during subduction

and collisional events. The preservation of a complete

Archean ophiolite sequence in the North China craton is

therefore of great importance for understanding processes of

Archean seafloor spreading, as it is the most complete record

of this process known to exist.

Despite the apparent abundance of partial dismembered

Archean ophiolites, no complete and laterally extensive

Archean ophiolites had been previously described from the

geologic record, leading some workers to the conclusion that

Archean tectonic style was fundamentally different from that

of younger times. The presence of a complete Archean ophiolite

suggests that Archean and similar younger tectonic environments

were not so different, and that seafloor spreading

operated as a planetary heat loss mechanism 2.5 billion years

ago much as it does today.

See also ARCHEAN; CRATON; OPHIOLITE; PRECAMBRIAN.

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