China's main stock index briefly broke through a key barrier on Wednesday on blue chips before nose-diving into negative territory.

The benchmark Shanghai Composite Index, the most widely watched indicator of the mainland's stock market, sailed past the psychologically important mark of 4,000 at 10:49 am in morning trading.

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That marks a gain of 49.53 percent so far this year on top of a 130 percent rally in 2006.

However, the Shanghai index then plunged before closing at 3,926.447 points at the end of the morning session, 0.60 percent down from the previous session.

Some blue chip stocks showed strong performances. China Unicom, the nation's second largest wireless operator, gained 8.84 percent to 6.28 yuan per share.

Bank of China rose 6.01 percent to 6.00 yuan, while Industrial and Commercial Bank of China was up 4.38 percent to 5.72 yuan.

Analysts said the market may undergo drastic vibration after the index breaks the mark.

An influx of investor cash after China's week-long May Day holidays helped push share prices further up when the markets reopened on Tuesday.

Analysts said the confidence was spurred by gains on overseas markets during the seven-day shutdown of the Shanghai and Shenzhen markets.

The Shanghai Composite Index closed at 3,950.01 points on Tuesday, up 108.74 points from the previous close of 3,841.27, on a turnover of 207.4 billion yuan.

The component index on the smaller Shenzhen Stock Exchange climbed 4.93 percent, or 535.24 points, to close at 11,401.12 points on a turnover of 105.7 billion yuan.

The central parity rate of China's currency yuan, also known as Renminbi (RMB), hit new high at 7.6951 yuan to one U.S. dollar on Tuesday.

Analysts warned of potential stock bubbles as expectations of RMB appreciation would draw hot money from abroad into China's equity markets.