well, according to this site, Iowa is yes:
Federation of Metal Detector & Archaeological Clubs Inc.
Albeit at "beaches" only. Doh. But you have to bear in mind how lists like this one, came about, to begin with. Here's how: someone asked. Sounds logical enough. I mean ..... who better to ask, than the state's themselves? So for example, when lists like this first came about (a book by a guy named "Grimm" wrote "Treasure Laws of the United States decades ago), he did it by the following method: He merely sat down, xeroxed off 50 letters, and sent them to 50 states park's dept's headquarters, asking "what are the laws regarding metal detecting in your state's parks?" Then he would take all the 50 replies, compile them in his book, and ...... presto: In theory, you could merely take that book with you when you travel, and know ahead of time if you could, or couldn't, in any particular state's parks. And if someone hassled, you, you'd merely pull out the book, and show them the law. All this sounds logical enough.
But the odd thing was when books and lists like that surfaced, culled from people asking, there was the odd pyschological effect of ....... sometimes getting a "no", simply because the desk-bound clerk, tasked with answering this letter, applies something silly. Like cultural heritage verbage, or whatever. I mean, ask youself, if YOU were the bureaucrat answering that letter, what do YOU think the "easy" answer was going to be? Afterall, there are, in ANY state, admittedly some historically sensitive monuments, right? And no one would argue with being able to snoop around those, right? But then there may also be legions of other parks which no one ever cared, and they weren't an issue. But when answering a question like this, you simply can't go into detailed minutia like: "no at these 10, but yes at those 20, and yes at this one, except on the north end where the cabin is..." blah blah blah. So apparently the "easy" answer was "no" most of the time (as you can see reading that list).
Oddly, there were a lot of states on those lists, when the lists started making the rounds, where ..... quite frankly,..... no one had ever had a problem before. So when these lists came out, you had old-timers scratching their heads saying "since when?". See how the self-fulfilling pyschology works? It's the old "no one cared, till you asked" routine.
And I can tell you for a fact, that some of those states with "no's" (or limitations such as Iowa), may in fact have state parks that are detected, and no one continues to care. Or perhaps you can go till you're blue in the face, and receive nothing but a friendly wave from the ranger. But I bet if you were to walk up to that SAME ranger and ask "can I?", that perhaps he'd have to check his rule book, call state capitol (as perhaps it never occured to him), and tell you "no".
Moral of the story?